For ease of use you might go with electrically powered engines.
I've listed the American made ones with which I have experience.
Jensens are great engines and are still being made. You can
easily find the common ones on ebay starting at under $100 for
a nice runner like a #35. As a bonus, you can buy brand new parts
to solve most all problems and Andy is there to help. Jensen
also offers 'fueled' engines, but the electric ones are the coolest.
Empires are robust and you will find most are pre-WW2. Condition
matters, but you will find they go between $50-$150. If the
heaters are burned out there are alternatives and most other problems
are solvable with a bit off toil and luck. Sometimes, you may have
to hunt for parts.
Weeden also makes electrically powered models but they are not
as durable an engine as the Jensens or Empires are. That said,
they are good runners.
There are a few others but the brands I mentioned will give you the
most pleasure and fun for the time and money spent.
Good Luck,
Wayne