Intro and questions from New England

Hi Folks – I’m trying to sort a few things out (about biking, that is) and this seems like the place to start. I live in western New England, which means it is sort of flat in the valleys but the hills tend to be frequent and steep. I live a few hundred vertical feet up one hill and I find myself not riding from home very often because of the climb to return home. However, I enjoy riding on flatter ground and my wife and I go out often on a paved “rail-trail” near home on a 21 speed tandem cruiser. I have a Specialized hybrid with 700c’s that I might want to motorize at some point but I’m thinking I might pick up a tag sale mountain bike to experiment with first. I’d like to pedal most of the time but use a motor for a boost on the steeper or longer hills. I’m willing to pedal along with it, and 12-15 mph is plenty fast for me. It seems Lithium/electric hub would be the gold standard but that is not within reach at this time. SLA would work but is still pricey and heavy. 48cc chain drive is light, simple and priced right, but it looks like the rear wheel drives the chain and sprockets when the motor is off or the clutch is disengaged, and that seems like it might waste some of my precious leg power. I’m thinking a gas engine friction-drive mounted on a pivot that can be lifted off the wheel when not in use would suit my purpose. Do kits exist for this or do I have to cob something up? Are there advantages to front/rear wheel with friction? I have searched the site and advertisers some but it looks like people mostly build their own using BMX pegs, etc. Thanks for any help

Thanks for the info and links - I'll look it over