If you're going to main engi, MAX OUT THE REPAIR TOOL AND AMMO PACK. This is of double importance if you're maining sundies. After that, I'd max out flak armor, then invest in a new carbine (assuming you're NC, I'd go with the AC-X11). The guns are mainly sidegrades, but after maxing those 3 things, it's a good way to go. As far as sundy certs, max out blockade and invest a little in mine guard as well if you're getting hosed by engies dropping in with tank mines. Kobalts are a cheap way to make your van a battle bus (as are the default basilisks considering the gunners have aim). Don't forget to help out those maxes as well! Huge amount of support certs to be earned there if your fragging isn't up to par.
I'm a squad leader in arguably THE BEST outfit in the world (NUC), so if you need any pointers with pretty much anything, just PM me, and I'll try to help out when I get a minute.
ETA:I've also spent much more on this game than I'd care to admit...that said, best deals would be membership, followed by the heroic boost on sale this weekend. Boosts are the only true shortcuts, as most of the guns really are sidegrades. Let me know what you're considering for purchase, and I'll give you a pretty good opinion.