First off Matt, mechanics has always been a hobby, only since the kids are grown have I chosen to use my abilities professionally, and that's on motorcycles. There is no doubt in my mind that you will give them hell, here are a few things that may help.
The fuel pressure sensor - Remember GIGO from earlier days of computers? (garbage in, garbage out). The cars computer will operate the engine in either a default mode or modify ignition timing, injection rate, or shift patterns accordingly when receiving incorrect input. This results in a decrease in performance and economy. Additionally, depending on its programming, the FI (fault indicator) or similar warning light may come on forcing you to have a diagnosis run. If this type of error code is not stored (and it is of a type that might not be) then the shop may have a tough time finding the problem. A good shop/mechanic won't guess at things, but with others the costs to you aren't their concern.
The timing belt - because they call this a serious concern the motor is probably an interference type (copied from cars.com)
6. Broken Timing Belt...Cost: $1,500 to $3,500
The lowdown: There are two kinds of engines: interference engines and non-interference engines. Or, as we refer to the interference engines in the trade, motor wreckers.
An interference engine is actually a more modern engine design, where the valves open wider and into the path of the upcoming piston. This lets the engine breathe better, giving it more power and better fuel efficiency. It all works fine as long as your timing works fine — when the valves are open, the piston is down, and when the piston comes up, the valves are closed and out of the way. If your timing belt breaks or jumps a notch on an interference engine, the piston smashes the valves, and you need a valve job ... at least. That's why it's crucial to change the timing belt at the recommended interval, before it gets anywhere near the point of breaking.
On a non-interference engine, a broken timing belt will leave you stranded, but it won't crush your valves. You can ignore the timing belt change on one of those engines if you don't mind getting stuck. On an interference engine, you're rolling the dice on a large boat payment for your mechanic.
A Mazda dealer has the car, not a Nissan - so either they took the car in on trade or picked it up at the auction. If it was at the auction they have their own money in the car, even though most likely they got the car dirt cheap. They still need to move it or return it to the auction. It looks like they are going to lose money on this one regardless. The fear you must put in their heads, especially if they made the mistake of picking it up at the auction, is how much they are going to lose, not if. Talk to the finance manager before committing and he will want that car off the lot asap.
Sorry I can't help with the dollar figures, it seems like you're at you best in that arena.
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