From what I know (now) at 71 is that bikes are for purpose. Like you, I prefer to find chassis I can work with in an evolutionary manner. I have a few bikes and have sold a bunch over time. Am also doing an eBike project. Craigslist (CL) is your friend. Mountain bikes are measured somewhat differently than road bikes. Hybrids somewhere in the middle. I suspect you may want to build a "gravel" bike. An efficient do=all ...

So what inseam? What arm length?

I'd start with a Specialized Hardrock Sport - an older one. Prolly $75 off CL. Get it serviced so it works and ride it around some. Think about the changes you want to make? It's a longish chassis, so moderately easy to make fit. Change stem and bars as needed to get the right reach. Change OEM seatpost for one with tilt adjustment so you can use aftermarket saddles a d slide to best position. Now you have basic fit

I am doing one. Nicely reinforced rear triangle so it will take rear hub motor torque well. Light'ish frame and some OK part like wheels. Over time, loose the low rent front fork in favor of a nice used air fork (in my case Maintou R7 Elite) which will loose over 3 lbs off the initial weight and transform the front end. Frame is roomy enough to take plus sized tires easily enough. So you can experiment with tire combos that suite your riding area best. I like Schwalbe Marathons for general trail riding.

Point being you can fit a medium sized mountain bike to a lot of bodies with aftermarket parts and thought. The parts are not horribly expensive unless you chase weight weenie gods. And in a few iterations you'll have a very comfortable bike that will do a lot.

I have seen these with high rise bars and drop bars doing duty as cruisers and as gravel bikes. It's up to you