Off-camera flash, radio triggers, starter options??

I've had my A7II for a few months now and tried to investigate options for an off-camera flash setup without spending big bucks Not getting very far; there don't seem to be a lot of options that give TTL metering on the Sony but that is really a secondary concern for me personally - I wouldn't mind a fully manual option to start off with. I have never owned a flash other than the built-in flash on my old NEX-7 (which I rarely used) but I would like to learn how to use a flash effectively.

Having followed a few online tutorials, I would like to buy an inexpensive radio trigger and/or a remote flash cord together with a reasonably powerful but not huge flash unit. As TTL is not essential, my main question is whether flashes and accessories designed for other manufacturers systems (Canon, Nikon etc) work OK from the Sony hotshoe?

I mostly take shots where I have plenty of time to experiment with exposures - quite a lot of macro and close-up shots and the occasional portrait - and photography is purely a fun hobby for me. My budget is not huge.

What would you suggest as a reasonably good budget solution?