I had a 10ft livingston a few years ago, and I'll tell you this...
If your boat can handle a 6 horse...get a 6 horse...
IIRC, mine was rated up to 10hp and I threw on a 6hp I got for a song. It did alright, but I complained regularly about it's giddy-up. I wish I would've gone 9.9hp...and I'd be much better off now, with my currect setup as well!
Livingston's aren't the lightest boats, and once you add an adult or two (or a kid or two) plus gear, you'll wish you had more motor. You can always throttle back a little to slow down...ya can't get any more that wide open with a smaller motor!
Now...my 90's 6-horse 2stroke evinrude is on my 12ft harborcraft aluminum-V and it'll plane with just my 210lb self, and ALL my gear. With 2 adult men, it'll still scoot up the skagit just fine for pinks and silvers.
I tried a 5hp Mariner on the same boat, and it wouldn't even plane with a 160lb driver and no gear.
So if your next step is something similar, you'd probably be happier with the 6hp at least to save a few bones, and maybe *just get by* with the same motore on a larger boat.
I see late 80's to late 90's 6hp 2 strokes regularly for $300-400 in good condition.
I'm a fan of mine, because its a twin cylinder, so it's fairly smooth...still very light and easy to carry...You won't find any twin cylinder 4 or 5hp motors.
As for the 2strokes being banned...
Yes, it's coming...but if you're in the market for a sub $500 used outboard...who cares. If it lasts a season or three, it was money well spent IMHO. Use it as long as you can, and use the trolling motor when you can't. Save the big bucks for an outboard on your NEXT boat...
Wow, it's been a long day...did that even make sense?