Your flash quite manual, so I dunno if it's too complicated to use

I have an old Sunpak one as well (relegated to storage) that I used with my dad's old Nikon FM before. Basically the dials/switches behind are for you to inform the flash unit what sort of settings you have on your camera, and hopefully it will control the output well enough.
Forget all your auto-exposure (P,A, S modes), i-TTL stuff, because this is pre- all that.
If I were you, seriously I'd relegate that flash to storage. Don't beat a dead horse. Technology has moved the goalposts. i-TTL is really a god-send for hobbyists like us who are quite 'blur' about flash photography and how to balance flash output with ambient lighting.
I second the suggestion of Nissin Di866 as a budget alternative to SB900.
But my #1 choice is a 2nd hand SB600 for general use. It's smaller (though less powerful), but still sufficient. I got mine for well under $300.