Hi there. I am new to the 1/8th scale Buggy / Truggy scene (6years exp with 1/10th scale scts and tamiya ta05 gear) and recently picked up a Mugen MBX6 - Eco from a friend for 500$
Here are the specs

Mugen mbx6 Chassis
Tekin RX8 esc
2050kv Tekin Buggy Motor
Three 4cell 50c Gens Ace 5000 LiPo's
+ 120$ in random spare parts ( Some aluminum some plastic)
I'm wondering if there are any parts that are a mandatory upgrade.
Thanks in advance!

Nope! That's the best thing about The mugen eco. Upgrades aren't needed.

Maybe ceramic bearings but that's just for more speed other than that, that's probably the best buggy out.

A arms... Buy PLENTY of sets. The problem is the A Arms are rather brittle. I campainged a MBX6 Eco Last year and won more races than I lost with it. Even TQed a couple of times. I did snap A arms often though, even kept me from placing on more than one occasion. This is with easy , all be them fast, landings Also, check all chassis screws. Make sure they are not frozen. I love the kit. Best handling I have ever driven.

Screw kit !!!!!!

I swear almost every screw fell out of both my ECOs lol.

What about RPM replacement A - arms , Have you tried those? Or do those have too much flex

There are no RPM arms for the Mugen. I've heard of some people boiling them for increased strength.

Sounds like you got a good deal that buggy really comes ready to race out of the box doesn't really need upgraded parts maybe some up high priced ceramic bearings and maybe some extra parts just in case but all in all good job on that Buy

I want to get one my self