Well L96 mb01 sniper

You would also want to upgrade the hop up unit, barrel and hop up rubber as well. I used to run 0.4-0.45g bbs

I bought the upv version which is 2.4-2.5joules so should i buy 0.36 or 0.40.?

Don't waste money on the generic 500fps upgrade kit, if possible better off buying quality parts.

A stock plastic trigger box will last a bit at "500fps' aka 2.3 joules. (Heard some people's have lasted years others not)
The generic metal & e&c ones should last a decent time if treated right.
Aftermarket ones will last forever and have better trigger pull as well.

BB weight depends on barrel and hop rubber, stock rubber might not be able to handle heavyweights well. With a stock 500fps barrel I would try a .4 but you may need to buy a new hop chamber,barrel and rubber if the rifle won't hop it. Or S/R hop it.

Usually the MB01 comes with a 500mm 6.08 barrel, so it be best to upgrade that.

If you want your rifle to shoot further, firstly upgrade the hop up chamber, hop up rubber and inner barrel, to atleast a 6.03 diameter.