With regards to the M249 there are the G&P (uses V2 gearbox, Classic army (proprietary gearbox), A&K ( clone of Classic Army) Ares (Clone of the Classic Army gear box except the nozzle is off set and uses a proprietary tappet plate). anything else is mainly just a rebranded A&K. The classic Army clones have AOE issues and strip pistons( but are easy to work on) RPK uses a standard V3 gearbox, A&K PKP uses the Classic Army gear box, MG42 (proprietary gearbox, the gun it's self is fragile). Classic Army Stoner 96 (basically M249 internals). Oh forgot A&K M60s again ( clone Classic army gearbox same issues).
Personal choice would be G&P MK23 but they are hard to get hold of and over budget.
Land warrior have some relatively cheap A&K m249 in the moment and have decent parts support.