Jon:
Congrats on inheriting a colony

. This will also be a great learning experience for your kids. Later on they may have their school friends come for a Martin event. The more younger people that get introduced the better.
You need to get moving on getting housing up. Poles will need to be cemented in the ground.
I'm central PA ...about 20 miles west of Williamsport . I think I may be the furtherest colony, in north central PA, as far as I know. Last year my colony had to endure 11 straight days of snow, sleet, freezing rain, cold right after they arrived. They were sitting in snow on the balconies. I had to suppleent feed out my back window, as I had bronchitis.
If possible contact the previous owner & ask how many pairs he had. Then decide how big your want your colony to be. Put up enough housing to cover that many pairs plus a few extras. The neighbor, he gave the houses to, will get any overflow. I limit my colony to two T-10 houses. That's all I can care for, as I live in town & have health issues.
I would suggest sturdy square poles with winches to raise & lower the housing & gourds. They do better than round poles during strong wind storms. The PMCA store has all you need. I would suggest changing the cables to stainless steel. Lowes sell stainless steel cable. You'll find it in the section where they have the nuts/bolts/screws/etc. Usually on the bottom shelf near the floor. They will cut it to length for you. It will hold up much better than the regular cable. We marked the cable with paint where to stop cranking when raised. Made it easier than looking up the whole time.
I prefer balconies with railings on my houses, but many don't use them. Railings help to keep chicks from falling off or getting pushed off.
As far as entrance holes....they will adapt. I changed houses after several years & then changed entrances as well.
I can't offer suggestion on owl guards, as I don't use them.
Get out & gather white pine needles now, so they can dry well. Lots of them. You'll need several plastic grocery bags full. It makes a great nesting base & keep the Martins warm & dry. Place a big handful in each nest & try to make a nest bowl towards the back. Straw works too, but it holds water. I use cedar chips on the bottom, then a handful of pine needles. I place straw & thin twigs, cut 3-5 inches from bushes, out for the martins to gather for nest building. You don't need to do this, but as I live in town it keeps my colony off the road. Also make sure the floors are not slippery. If they are you can add 1/8" piece of luan plywood as flooring. Drill a finger hole in one corner for easy removal.
Save & bake eggs shells now. Then crush them & put them out on a platform for the Martins. I have a frizbee attached to the top of the clothesline post. They need calcium for better laying. To do eggshells....Rinse well to get out as much egg as possible. I put them in a bowl covered with water until I get enough to bake. Drain off water. Lay on baking sheet & bake at 250 degrees for 20-30 minutes. Just until they are crunchy, but not brown. Let cool. Place some on paper towels & fold over the towel & crunch into small 1/4 inch pieces. Put in a ziploc bag. Once your colony has arrived place some out for them. Keep giving thru the end of season, as they will feed them to the chicks too.
Starling & HS removal.....trap or shoot. Never use sticky traps in Martin houses. Martins or other birds can get caught in them.
Supplemetal feeding is an option many use during no feeding times.....which is temps below 50 degrees, windy days, rain, snow, sleet, freezing rain, real hot/humid days & drought. Martins eat flying insects & they don't fly under any of those conditions. Many use crickets as they are easy to fling into the air. Others use meal worms. Buying local will cost a pretty penny so most order from insect farms via the internet. I think there's a post listing the sites most of us buy from.
Doing nest checks: You'll need a few things....a 5 gallon bucket for supplies. Rags, paper towels, nesting material to do nest changes when required. Paper/pencil to record each nest. I use a clip board with a nest check form I made on the computer. Also something to plug each nest hole later as chicks grow. Some use foam noodles cut to fit, other use rags with a long string tied to it. Watch your colony. Number each nest. Not long after you see them bringing in green leaves egg laying will start. You'll want to record how many eggs are in each nest. Then 16-26 days later hatching starts. Bad, non-fertile eggs won't hatch & are often pushed to the front. I do nest checks at least 2-3 times a week, more if weather is not good. I also tap the pole before lower to let the adults know the house is coming down. This keeps them from getting too scared, fleeing quicky & taking eggs or a chick with them. Every now & then a female will stay on the nest. So open doors slowly.
As the chicks hatch so does blow-fly larva. They crawl onto the chicks & suck their blood. A chick can become very weak in a matter of 2-3 days. Once they reach that point they are too dehydrated to survive. So nest checks are very important at this time. Remove any larva from each chick & change the nest out. Check all over the chick....feet, legs, under wings, neck, etc & remove the larva & smash them. Some chicks can be saved by supplemental hand feeding crickets dipped in pedilyte or gatorade. NEVER give a chick water or any liquid other than a cricket or meal worm being dipped in it. There are posts here about handfeeding, etc. Always place the chick back in the nest stomach down & towards the back of the nest.
As your Martins begin to arrive go out & talk to them. Say hello, welcome back, etc. Don't get too close at first. They will adapt to your moving around your yard. Also never lower the houses after dark. If a storm is due lower the house before the storm. The Martins will be OK. I lower mine to about the center of the pole.
Listen & learn their sounds, calls, songs. You'll soon learn when they are just singing or putting out a warning call that a hawk is close. They are super aerial fliers, so you'll have a show each night as they come & go before dark. They are noisy at this time.
You'll want a camera & binoculars too. This will become an obsession. Do your best to get your wife interested. She's got her hands full with a newborn, and 2 other young kids, but seeing baby chicks may sway her.
You'll learn as you go, so keep coming here & asking questions. Someone will answer.
Make sure you come back & post how this all goes & how many pair you ended up with.
Best wishes on becoming a Martin landlord

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Toy in PA