I love my flatline, I wouldn't listen to what anyone here says as far as the flatty is concerned. Go find someone who has one and ask if you can shoot it. That's the ONLY way you will know if you like it. it's not for everyone. I have one and find it's not as paint picky as everyone says it is. ANd most people who critisize it either don't have one or haven't given it a chance.
As for other upgrades I have a stock and like it. You will want some sort of offset rail if you are putting optics on it. (I would suggest the largest red dot scope you can find. Not the square crossbow ones you find at walmart.) You will want the offset even if you DON't get a stock because the hopper is in the way.
You also have an RT, so doing HPA (Nitro) would be a big improvement to your rate of fire. And Palmers pursuit shop makes a GREAT reg called the Palmer stabilizer.
This will help even with CO2 to make you consistant and make your RT really sing.
Good luck and happy modding. !!
(Once you start you can never stop!!)