I've been detailing my cars for years but only recently got into Machine Polishing so perhaps my thought process will help you. I will only be polishing my two cars (2007 Passat Wagon and wife's 97 Escort Wagon-soon to be replaced) and perhaps a relative or friend's car now and then so I wanted to keep costs reasonable but make sure I purchased everything I'd need to get the job done.
I read incessantly for a couple of months on this and other forums and watched videos on suppliers websites, forums and youtube and am really greatful to all the super advice given by others on these sights. There are hundreds of ways to do this, but this is what I culled from everything I read:
The Groit's Garage 6" is the most powerful of the DA machines and has a lifetime warranty so that was a no brainer for me. I bought the one with a 10' cord and got a 12' extension for $7 at HD (16 guage as recommended by Groits).
I decided to use HD Uno as a polish as it would keep costs down and shorten the learning curve--UNO cuts, polishes and finishes based on the pad, speed, pressure and arm speed and does this with no dusting. For $18 you get enough to do 4-6 cars and a free bottle of POXY sealant to boot. With other products you could easily spend $50-$100 getting a compound, polish and finishing polish and then have to learn how to use each one.
Everything I read said 5.5" flat pads were the way to go -- most effective, easier to control and cost less. So I decided on Buff & Shine pads (UBERs are the same pad) because their Velcro is recessed 1/4" from the edge of the pad for a little extra safety when polishing near adjacent paint and protrusions like mirrors etc. B&S pads appear to be the exact same foam as the HD pads for use with UNO and POXY so I purchased 5 orange, 4 green and 3 red pads. Mixed six packs of 5.5" pads can be had for $35 - 6.5" six packs are $50 -- so you save enough on the first six pack to get the 5" backing plate. I also bought a 3.5" backing plate and 2 each of B&S 4" pads in corresponding colors for small areas (also got 2 yellows if I run into any deeper scratches in isolated areas) Cleaning my pads with Dawn Power Disolver which you can buy at Walmart for less than $3--plenty of pros out there using this with no problems. Later I'll add MF pads if I feel I need them.
Once I had everything there was a real urge to just jump right in and do everything but I decided to take it slow. I gave my car a coat of POXY and my wife's car a coat of DG 105 with red pads (these pads now become dedicated to these sealants) to get a feel of using a polisher since you use slower speeds to apply LSPs and I'm glad I took this step. I got a sense of how to keep the pad level and how to handle curves in the surface and realized I can control the GG much better without the handle. Also realized I'm glad I got the pads with the recessed Velcro especially around the roof racks on these two cars and that you still need some small foam pads to hand apply product in some areas. So you will need something to do some hand polishing also--HD has DUO pads that have orange foam on one side which will have some cut with the UNO for very small areas.
For polishing I'm lucky in that I have my wife's older car to use for practice (maybe you could find a friend who has an older car who would like a free polishing). I divided the hood into 4 sections each about 20" X 20" to do 2 test areas -- green pad with 4 passes at speed 6, 2 at speed 3--green pad with 6 passes at 6, 2 at 3, orange pad with 4 passes at 6, 2 at speed 3 and orange pad with 6 passes at speed 6 and 2 at speed 3--all with UNO. To my delight all came out great, but with closer inspection you could see the improvement with each section. Each section finished down well enough that I might not have to do a second step on her car before applying an LSP since it will soon be replaced, but most importantly gave me more of a feel for polishing and handling the GG RO.
All in all I'm really pleased with my decisions, but now really anxious for the weather to warm up a bit so I can do both cars completely.
Read as much as you can and watch as many videos as you can. There are short videos on the HD sight with each product (UNO and POXY) and there are allot of good articles and videos here:
Articles by Mike Phillips - Auto Geek Online Auto Detailing Forum
Also, if you're not in a big rush Amazon sometimes has the GG on sale for $80 for just the polisher or $100 in a kit that sells normally for $200--watch the Gold Box Deals and be ready to act fast as they usually sell out in 3 or 4 hours.
Perhaps others with more experience will chime in with additional and alternate suggestions.