If your goal is to learn how to gunsmith on an AR, get a stripped lower, a lower parts kit that doesn't include the trigger, and then go to town. CMMG calls this a "gun builders kit."
Note here is that you'll have to also get a separate pistol grip and trigger guard, as those are the other 2 most commonly upgraded parts.
All that being said, if you want a 5.5 or 6lb trigger, there's no reason to spend the bucks on a Geissele. Just use a milspec trigger.
Premium triggers exist because of their safety and customizability at lower pull weights. A 5.5 lb trigger is stock-ish. 6 is heavier than stock.
Geissele and Timney are the brand names of the AR trigger world. If you're looking for bang for your buck, check out P.O.F., and CMC.