This shop usually does custom work for higher end race cars or collectibles.
So how much do you trust them not to get bored with this diversion if it doesn't go well? They do high end stuff, making a Frankenstein Xterra is going to be fun for them how long exactly?
Are they going to be ready to jump in and fix it if this cobbled together mess doesn't wear as intended and you start burning through bearings and chewing up gears? You think they are going to bring their best work for this project? Are they going to say "Aww sure man, we will get you in before we do the Rolls there....."
Hey, it could go smooth as silk and last forever. I'm not saying it wont. Do you gamble much?
Cheap now = Expensive later
It sounds to me like a setup to get yourself screwed. The shop could be above all reproach but still when you leave with it, it's all on you. You'll be stuck with this oddball Xterra no other shop will touch, and since you are having a shop do it I presume that means you are not equipped to handle such a task yourself. Also, are you going to be right in there with them personally versed in what all they did to make such a conversion work so you can fix it later presuming you are a mechanic, self taught or otherwise?
It sounds like you can't afford an Xterra or don't want to travel for one. I don't say this to be mean but having a shop hack one together for cheap means you still can't afford it
. Drive some reliable but beater vehicle and save your money, buy what you want later and get something that won't milk you dry.
Frakentruck is fun if you got a shop to piddle in and time to goof off with it and something else to drive when it craps out, if you are concerned with reliability, you don't want Frakentruck!