Must admit i know bugger all about mccullog chainsaws.
Is it from a local dealer/service centre or some internet thing?
if from a local service place or if there is a local place i'd ask there, if no local service places i might think twice
I do tend to think if a localk place services a cheaper brand of saws they are standing by that brand and u have some back up spares for it
As for chains and bars again havent a clue wot gauge they run on could easy be 1.3 pico chains or 1,5 or 1.6 mm more standard 325/3/8's grooves, it will also depend on the bar mounting bolts and oil hole wot bars it takes.
Go on a site like chainsawbars.com? just type in the saw and a host of bars will come up ( really good site to deal with) but u will need to know the chain pitch/type ie 325 or 3/8's or a pico low profile version of them ( likely 1.3mm bar groove thickness)
Must admit i've got most of my small saws on the 325 1.3mm (0.050) chain now cuts well althou slightly dearer than standard 325 and u need a different bar for it
The only other thing i look for which i don't think anyone else does is wether clutch is external or internal.
My pet hate is external clutches!!!!

Don't think makes any difference to how saw run but if and when u get ur bar nipped in a log with an internal clutch ( most stihls i think and huskys over 70cc) its a doddle to take ur bar and chain off and cut ur bar out with a spare bar and chain, with an external its just a bloody nightmare and usually ends up in a long walk for a 2nd saw.
If ur always in ur yard/close to ur pick up extra tools not really an issue thou