Welcome!
In my area, the easiest trike to aquire is a 110-200cc hardtail honda. They're the most affordable to buy, and being hardtails they're the cheapest to maintain. All? Of them have dual range (high&low).. so even a 110 could pull a trailer around your yard.
A big red would be the prime choice, but, will cost 2-3x what a hardtail would (in my area anyways) and like Jim said.. will need work. When it comes to powersports.. USED + 35 years old usually = clapped out. Youll pay a premium for a running machine then have to put a bunch of money into it.
1980 Honda 110-125 lifan manual clutch *in progress*
1985 200x - building peice by piece