A few additional things to consider are while a bike can be had at Walmart for under $100 new - the rims are treacherous. While the rest of the bike may be fine (get front brakes if it's only got a coaster) the wheels aren't really up to this sort of thing and will probably need replacing - I'd hunt up a good used bike if money is an issue.
I would also suggest picking out and purchasing an engine kit first
, that way you'll be sure to get a bike it fits well on - most problems encountered during installation are a result of the kit... almost
fitting and needing some fabrication/modification.
As for which engine, well that's up to you and your budget - the rack mounts the easiest, the in-frame somewhat cheaper and a better ride. Remember - size isn't everything if you've no need for speed - sprockets can be changed for more hauling power.
No matter what you choose - pick a reputable vendor as you may need their patience, customer service & parts. Saving $20 from some random ebay seller may well end up costing more than you think