The best thing you can do to prepare is to read as much as possible. Both here and other places on the net. You will be better able to make an informed decision.
Are you handy? Like to fiddle? Do you have patience? Nitro may be for you. Nitro takes more maintenance, and can be a fair bit more frustrating, especially to the beginner.
Electric may be the way to go if you want to avoid that frustration. They aren't maintenance free, but they are easier to get running and maintain. Electric is a good way to decide if you want to progress further in the hobby.

A 1/10th stadium truck, either electric or nitro is a good place to start. They can run on a fairly wide variety of surfaces, are simpler and easier to maintain, and can be had at a decent price. They are also a good place to start learning to drive. The lessons you will learn maintaining a ST will transfer well to whatever you get next. (Mark my words, there will be a next if you stay in the hobby)

Monster trucks and 1/8th buggies will cost you more, and be a bigger challenge for you to consistently keep running. If you're sure you're gonna stick with the hobby whatever problems you face, go ahead and get what you want. A monster will give you the most options of where to run, with a 1/8th buggy not far behind. Both can be raced competetively (as can the stadium trucks) if there's a track in your area.

Don't stop reading once you make your choice and get your vehicle. There's piles of information and just about every imaginable problem has been covered around here. Knowing common problems and how to deal with them ahead of time is a good way to avoid frustration.

It's a good idea to go with a brand that your LHS supports. Make up a list of vehicles you're interested in and find out what they stock parts for. It's a bummer waiting a week for a $2 part to come in. It's much better when you can just pull one off a rack and walk out with it. Your LHS may or may not be a good source of info. Load up on your own knowledge and find out for yourself. Having a good relationship with the employees is valuable whether they know their stuff or not. I personnally got lucky and have a great LHS staffed with guys that bash and race and fly (Kudos AVF in London). You said you felt the LHS was pushing you to the most profitable trucks. That may be so, but they may also be pointing you to what they have good support for, and/or run themselves. Find out. What exactly were they pushing you towards? Ask around here if they were giving good or bad advice.

It's funny that buying a toy can be such a daunting task. Arm yourself with knowlege and happy hunting.