There is gas and then there is nitro. Gas is used larger scale's and above and nitro is used in 1/8th scale and below. You will be paying a lot of money for a gasser so your money range will be in the nitro. Ok so your looking for a nitro, nitro the fuel is a mix of alcohol, nitromethane and 2 stroke oil. Depending on the fuel it will cost any where from $18 a gallon to $30 a gallon but a gallon of fuel will last a long time so dont get worried.

The capablities of switching bodies in not a problem and 30mph is also not a problem. You want a nitro MT so your looking at either a Traxxas T-Maxx, Revo a Savage x Team Losi LST or LST2 or a Team associated Monster GT.

Personally I would go with either the T-Maxx, Revo or Savage. Thay all have a ton of after market support but the thing that will get you is there price. For them you will or she will have to go to ebay. Since your asking about it now (which is a darn good thing) you or her can search ebay for deals. Since it's for christmas you can be patiente you or her should be able to find some great deals on ebay.

HAHAHA almost for got about st's. You can get some 4wd st's. I believe HPI makes a truck with a ford body already on it. (I may be wrong though) and they are less expensive. You can get a new ST for around $250 and be able to get fuel and things of that nature with your extra money. Ebay is also a great place for them as well.