How anal are you about detail? Do you want to build an actual MK18 if so which one - Mod 0 or Mod 1? Block I, block II?
How much do you want to spend?
Building a MK18 to "proper" standards is a deep rabbit hole that will empty your wallet and drive you mental.
This is a good example of how far that rabbit hole goes by the way...
http://miairsoft.proboards.com/thread/46829/build-mk18-mod-1
As far as bases for builds there's a few if you want to go down the ready made route. The G&P ones are good
https://www.fire-support.co.uk/product/gp-m4-mk18-sbr-aeg-airsoft-gun, the Tokyo Marui Recoil one is good but expensive
https://www.fire-support.co.uk/product/marui-nextgen-mk18-mod1-airsoft-gun-aeg or there's the Specna Arms SA-A03 for a more budget friendly version
https://www.patrolbase.co.uk/specna-arms-sa-a03-mk18-carbine-assault-rifle but be aware it won't stand up to the stitch counters that will point out that the 10.5" barrel isn't right...