Outfittings make a huge amount of difference - they really are what you pay the money for and it's worth it too.
You don't really appreciate the difference the difference until you've spent some time a well-fitting boat but well, it's big. The club-spec Pyranha stuff is especially sparse, not even any kind of thigh-braces - you'd need to do a LOT of work
What's "cheap" anyway?
You used to be able to order the parts from pyranha (perhaps not any more as they're phasing out connect30) but they were really expense, something like €72 for thigh-braces, €50 for a back-rest, €80 for a seat, €40 for hip-pads. you get the idea, too expensive to be worth it. Making/taking from other boats will be crap for most parts at best and in some cases (thigh-braces) impossible (some parts might work - foot-rests maybe? but not enough).
IMHO - find another boat.
That Mamba you're enquiring about looks perfect (apart from the colour, but who knows, you might like lime-green, it'll match your paddles at least)and €600 is a for good price (The guy who's selling it would have got it for cost price and probably isn't too pushed about making a profit) - queue comment from peter about it having a big arse.