I had an old single converted by Bilenky. Wanted some other framework done to it, which took some of the sting out of paying for the new paint job as well. Flown with it a few times and it certainly is easier. Only downside is the cost and a little weight.
I think I'd scout some for a used coupled tandem before converting an old one as that would likely be more cost effective.
If you are going to the trouble of converting/repainting the frame then I'd certainly add the disc mounts as it won't be that large of an addition the cost.
Fork choice will depend significantly on the geometry. Not a lot of choices. Maybe look first a the current Santanas and then at the WoundUp.