I've been using a Marui P90RD for many years
Done lots of work upgrading it over time as well (
I posted a list of upgrades here along with some photos), although they do work great out of the box as TM stuff usually does - I recommend at least getting a MOSFET fitted to protect the trigger contacts though.
The sight on the red dot version works well enough but it's not for everyone as it is quite small - I get along with it just fine but you might not depending on how big you are & your face protection; I use a slightly modified ASG soft-sided half-face mesh mask along with some Revision Desert Locust goggles. If that's a concern the TR model with a larger optic such as an EOTech-style sight might be a better choice... the TR model doesn't look as good though.
As for the magazines, I currently use & can thoroughly recommend the MAG 170rd midcaps. The standard 68rd mags are fine too ? For a long time I used those as I dislike highcaps in general & nobody made P90 midcaps at the time. The highcap P90 mags are an interesting design but have a tendency to jam a lot & need a slap to unjam, although you can improve their reliability by opening them up and lubing the mechanism with some silicone oil.
There's also the possibility of using a STANAG adapter such as the one posted above, but just... don't do that ?
There are limited options for P90 mag pouches but the Viper double mag pouches work well enough. Also the Maxpedition Mega Rollypoly works great as a dump pouch for P90 mags.
For sling options, I think there's a lot more choice than there used to be so it shouldn't be too much of an issue to find something that works ok. I used to use a Stargate style P90 sling but now use a Magpul MS3 (along with the P90 side of the Stargate sling to provide an attachment point for it).
