Hi Matteo,
I'm putting another vote in for the FX Aid, I have the XL version (it's 6HP) and have been continuously impressed by the algos on offer, sounds fantastic as end of chain effect, probably the best value for the money and fits in your budget. The fact that it has so many possible effects, some of which combining for example a delay and a reverb make it an excellent choice for someone in your situation with no FX module in the rack yet, even if you add larger and/or more specialized FX modules, this one will most certainly retain its value in the rack...
Another choice to consider I think and also within budget is the Noise Engineering Versio thing (https://www.modulargrid.net/e/noise-engineering-desmodus-versio). It is less flexible than the FX Aid in the sense that one needs to switch firmware to change the effect, but on the other hand the CV control options is way more extensive, and the algos are more specialized I think. NE is very very good at making digital modules and this one is no exception I believe, I've heard excellent stuff in the demos of the Desmodus Versio (the reverb) and the Ruina Versio (the distortion), probably a better choice for those looking to integrate the effect as part of the sound design rather than just end of chain. But all this is from the demos I've seen, haven't actually tried the thing myself...
As always, research extensively before you buy and you'll be allright :)
D.
--- Voltage control all the things ---