Plaits is basically its own synth voice. You can dial in attack, decay, quantizing, etc from it. The same with Braids.
I think a couple of envelope generators would be nice and should fit well within a $500 budget... unless you go with something a ton of features.
A multi-function module would keep you busy for a while. Expert Sleepers Disting EX, Maths, Ornaments & Crime, would be nice choices. The Disting would give you the most features. Maths is great for envelopes and other modulations/utilities, O&C is great for CV modulations but doesn't do audio rate (like reverb, delay, etc.).
It's a bit of a broken record. But I'd go with the Disting EX. If you have $500 and that's all you'll have for a while, it'll give you the most bang for the buck... it can even do filtering. Plus you'll get $150 back in change.