From what you've said, we can't tell if Simon was ever properly set up and in good working order when you first got him. If it was adjusted properly then, "absolutely cannot move the pedals unless it's on one of the four lowest gears" would not have been true. And you really shouldn't have been riding this bike with brakes that didn't work. Since we can't see this bike, we can't tell you if it's worth repairing. But no bike should be ignored and ridden until it's dangerous and all it's parts are broken. Perhaps you are right that the frame is designed poorly and it doesn't climb hills well, but this conclusion may not be justified. It could be that the bearings are shot or totally out of adjustment in one of the hubs or bottom bracket or you have a bent axle or spindle. It might be that Simon will be a good commuter bike for you if you repair it properly, or maybe you've ignored it so long that the repair will be too expensive. If you are going to own a bike, you need to either take it to a shop periodically or learn to maintain it. Sorry if this sounded harsh. I hope it was somewhat instructive.
In looking for another used bike, here are things to avoid:
a bike more than 27 lb
steel rims
a one-piece crank
anything made in America in the 70s (the above 3 criteria probably exclude it)
bolt-on wheels
Things to check:
straight frame, no dents, rides straight and true
bottom bracket - drop chain to inside and see it it spins freely and has no play
hubs - spin freely and no play
rims relatively true (at least not bent and no short-period wobbles horizontal or vertical)
spokes under even tension
Brakes and gears can usually be adjusted so not as critical. Drivetrain parts (freewheel and chain) wear out periodically and can be replaced. If you try to ride on them forever, the bike will eventually ride like crap.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72