The Mongoose is a late 90's Kmart type bike, (Is that crank spider broken?), the Schwinn Frontier is a late 90's model made after they went bankrupt, I'm not sure what they were as I've never had my hands on one in person. The Miyata looks like a nice bike but its 2+ hours away and likely too light weight for my size. At almost 350+ lbs the old road bike wheels likely wouldn't be a good choice here on mostly dirt, or oil and stone paved roads.
For what it would cost to drive to new hope pa it would be cheaper to buy something closer for more money. The Frontier would likely need new tires, something less aggressive, and better suited for my weight. Maybe even a better set of wheels as it likely has single wall stamped alloy rims with cheap plain steel spokes. I've got a set of wheels off a Walmart bike in the garage from a bike I picked up at a yard sale last year, the spokes started to dimple the rims and pull through after less than a week. I scrapped the bike figuring I'd make a yard cart from the wheels. It was similar to the Frontier but newer with different brakes and a shock front fork.
The Trek is gone, (or at least the ad is gone now), but it had a number on the seat post. The bike was dark green with white or yellow 'spatter' throughout the paint. The wheels were aluminum with 700Cx(40?) tires. (About the width of an older middle weight bike). It had thumb shifters like the Schwinn Mesa Runner did. The bars, stem, seat post and cranks were alloy. It had cantilever brakes It was a very tall frame, I didn't measure it but it was noticeably taller than the Schwinn Mesa Runner. I looked online but didn't see one like it looking through 80's and 90's catalogs. One thing that stood out was the Biopace chainrings which made me think it was likely late 80's vintage. The frame had a tubing decal but nothing I recognized, just some OEM tubing sticker I didn't look close enough to pay attention to whether or not it was chromoly or some sort of plain steel. Quite frankly, that don't matter much to me. I'm not racing, and at my size, a few pounds here and there on the bike won't matter much.
If the Frontier was closer maybe I'd grab it and try to upgrade it but its still likely close to an hour away and its a small frame. I also remember seeing the model name in Walmart too but that bike is likely too old to be a Walmart sold bike. I don't think they popped up in Walmart till around 2001 or 2002.
I do agree, a cheap Frontier in the right size would be a good start but only if its super cheap. Also, buying an unknown like that means it will have to be completely gone through. That Schwinn Mesa Runner didn't need all that.
Out of curiosity, I called around to few bicycle shops to see what they would charge to do a complete tear down and relube of a bike, most said they don't do that sort of thing anymore, "Its not worth it, it would be cheaper to buy a new bike". One said they'd rather not be bothered but if I bought all the parts from them they would do it for $200 plus parts so long as it didn't require any 'wheel work'.
Two told me they don't do wheels, one told me that "Any older bike is likely not safe and should be replaced".
I stopped in a bike shop in PA a few weeks ago, I happened to be right next door so I walked in and took a look. When I asked about used bikes he told me to go to a flea market, they don't do used bikes, I then asked what kind bike he recommended and he told me I was too big for any bike that would hold me. He told me that the only bike he would sell me was a beach cruiser and I likely was far too tall to ride a 17 or 19" frame cruiser.
He all but flat out told me he'd rather not sell me a bike in a round about way.
I'm a big guy, but not so much just fat, I'm big overall. I measure 6ft 3in tall, have a 36" inseam, a 44" waist and wear size 16 boots. Most of my excess weight is gut weight.
On an upright bike, my knees get in the way of steering, on all bikes I have issues with my knees hitting my chest on the up stroke of the pedals and I have problems finding a bike that gives me enough leg extension to pedal comfortably. I had a 69cm frame road bike back in the day, it gave me great leg extension but the dt shifters were out of reach and the stand over height was borderline. I had converted the bike to flat bars and stem shifters so I could ride it. It lasted about a season before the rear seat stays cracked away from the seat tube and one dropout was working its way loose in the frame. I wasn't anywhere close to my current weight then, maybe about 250 lbs tops. I wouldn't even think about riding on 27x1" tires now.
At my age, I likely will never get back to my 20 something weight or size but I do need to do something for exercise. I'm far less active these days than i was even 10 years ago, partly due to my age, partly due to being retired.