One thing to bear in mind is that one of the key aspects of mechwarrior is that you don't buy your mech... you BUILD your mech.
Now, in this game, you'll be buying a chassis and stuff, but the whole notion of the mechlab is that you get to customize it to suit your needs.
That means that the actual mech you'll be buying, in many ways, is a platform upon which you can put stuff that you want. Each chassis will have different capabilities in this regard. They'll have different hardpoints, allowing you to carry different weapons, different engines, etc.
To start out, you'll probably just want to decide on your overall role:
Light - Scouting, harrassment, etc.
Medium - More punch than lights, less mobility
Heavy - Main force on the field. Can be providing ranged support for rest of lance, as well as brawling
Assault - Slow moving, capable of making a huge difference wherever they happen to be, but can't really move around that much. Large amounts of armor, but can be an easy target for long range support mechs.
Generally, if you have no idea what you're doing, I'd go with a medium mech. The hunchback's a tough little ****** too... I loved playing that mech in MW4 with a longtom.
If you want more info, given that the devs here seem to be holding some respect for the canon, I'd check out some of the old Battletech TRO stuff, which will give you information about different mechs, weapons, etc. as they exist in Battletech. This game won't be exactly like battletech, but it may give you some background on what exists in the battletech universe.