I don't know that I have all your answers but here is something.
It's hard to say on the 14" vs 18" because everyone is different. What I think is loud may be quiet to you. In my own personal experience, I installed a 14" and eventually got tired of the sound level while cruising at 75-80 mph. I swapped it for a 22". There is surprisingly not as much difference in the two mufflers as I would have thought, however, the 22" is quieter enough while cruising that I am now satisfied. As for the price, I'd say that's fair. I ordered my Maggie from Summit ($130) and had it welded in locally for about $25-30.
I've never replaced the y-pipe and there seems to be some debate about the gain. If you have seen the stock pipe then you know that the Maggie pipe has to be more efficient. But are you going to be able to feel the difference from the driver's seat? I don't know but my guess is no, you will not. Similarly, can't comment on the price as I've never had it done but best I remember there is a little fabbing and welding involved.
As for the tips, any tip is going to be a good tip as long as it is stainless steel. Your muffler guy should be able to fab up an "adaptor" if needed to make the tips fit. Mufflex sells a set of dual 5" stainless tips that are bolt-on for DIY installation. They are a little pricey at $190 but you would want to compare this to the price of buying tips and paying your exhaust guy to install them.
06-08 chrome clads w/33" Cooper Discoverer H/T Plus, Mopar tailgate spoiler, debadged door trim & tailgate emblems, 13" antenna, LED dome & maps, Magnaflow 22" SI/DO, 18% tint, "HEMI" hood spears, VHT tinted tails & third brake light, Lamin-x optic blue fogs, leveled w/McGaughys 2" drop shackles.