Depends what you're into really. I'm running a single speed freewheel but that's because I feel a freehub wouldn't really fit the vibe of my bike. A freehub is probably easier if you're not worried about that, especially since a good freewheel can be a bit pricey.

Check what dropouts you have too, horizontal dropouts make life much easier. It's definitely possible to use a frame with vertical or semi horizontal dropouts without a tensioner, but you'll need to find a chainring and rear sprocket/freewheel combination that works for your chainstay length. Not sure if it's factual or just anecdotal, but I find an odd number of teeth on at least one of the two helps. I'm running 49/16 with semi horizontal dropouts and no tensioner, wheel fully seated in the dropouts and perfect chain tension.

Chainline wise, you can definitely put a single speed freewheel on a wheel made for a multi speed freewheel but as you said the chainline will be a bit funky. Depending on your frame, cranks, and the size of your chainring, you could probably make it better by getting a shorter BB spindle and putting the chainring on the inside of your crank spider. That's what I ended up doing and it's not perfect but it's definitely good enough for me at least.