This is all person preferences, so take with a large spoonful of salt. Read the forums and find the pro's and con's of each decision.
Personally I have dealt with 3 different "brands" of motors so far. A gasbike silver slant, a BGF ebay motor and a Grubee gt5.
Of the 3, all have runs within my expectations. I would recommend a Grubee as a beginner motor, and they start around 169.99 minus shipping I believe. Shop around.
Check state laws and make sure of the engine size, most only allow the 48cc engine. Some states require licenses, insurance etc, look into it now so you don't get surprised later.
The bike can either be gotten locally or online, new or used. bikebuyers.com is supposed to be decent, but I have no personal experience with them. I went with a 7 speed cruiser bike from walmart.com on my second and third build. Much more comfortable than a mountain bike on long rides.
Mountain bike, cruiser, multi speed, coaster brake. They all have pro's and con's. I personally don't like the idea of modifying a coaster brake, so I recommend a multi speed bike with 2 brakes, front and back.
Save some money to do the recommended upgrades of the bolts, spark plugs, spark plug wire, and locktite. I recommend upgraded puncture resistant tubes with the slime of your choice, I prefer stans no flats myself.
As far as carbs go, I like the nt style over the cns v2. Nt is easiest to tune. All the other accessories are up to you.
in the end, make sure you don't spend all of your money buying just the bike and motor. There are alot of other things to buy as well. Headlights, helmet, speedometer, spare parts, just to name a few.
Make a shopping list of everything you "might" possible need and see how much you can drop in each category. You'll find out the motor will probably be the cheapest part lol.