I'd echo lots of test rides, etc.
Wobble can be partly the bike (my first bent wobbled when I got slower than about 5mph and I was told it didn't have enough "rake") and partly the rider. If the latter, it will improve or go away with time.
The stratus is a great LWB bent and the long wheel base ones with the lower bottom btrackets (pedals) ar the easiest to ride. Just jump on and go, almost no learning curve!
I have a Rans V3 which is somewhat like the Stratus except that the pedals are a bit higher and you are in a slightly more laid back position. Pedals are lower and you are a bit more upright in a Stratus.
Lots of good bikes for him out there!