The CA sportline would be a good choice, I'm pretty sure the Echo1 P90 is just a KS rebrand but, it'd still be better than getting the KS version.
If you do get to a P90, I'd advise getting the box mag so that you'll be able to use M4 mags. P90 mid-caps cost $15 and only hold 68rds, I haven't been able to find a box set of them either. MOLLE pouches for them are also $15 and only hold one mag.
As for the R4, it shoots a fairly low FPS but, it's inexpensive and magazines are very easy to get for it, I'm also sure that mags for it fit in Mp5 and I think M4 mag pouches, not entirely sure though. I'd consider it an AEP and I'd only use it as a sidearm..
Edit: Trinityairsoft has some
P90 Mag pouches that are capable of holding two mags, they also attach to your belt/leg/other places.