you will loose your adjustments for user selectable things and oil life monitor reset, but if you get the car how you want it before you change it you will be fine. the oil life monitor you can reset by turning the key to run and pressing the gas to the floor three times.
the sounds the car makes for doors, fuel, onstar if equiped goes through the radio and if you want to keep them you need to get the wiring adapter that retains those. if you dont care there are cheaper options. i got the non retaining dash kit and wiring adapter from ebay for 20 for my 07 monte, on my old monte, same as your car, i got the adapters free and the sound retention adapter from crutchfield. if you buy a hu from them they give you all the dash kits for free and the wiring for half. even half was over $50, now that ive done it the other way i think its nice not have the buzzers and junk.
as for what to get, cheap and touch screen nav dont go together. anything on crutchfield should be decent, thats were i buy everything, when you factor in the free adapters, shipping, customer service, etc. hu'* definately go by the saying you get what you pay for cheap is cheap, if you want more quality and good sound its not going to be a cheap unit
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock