I'll echo many of the sentiments that have already been expressed: There's a discrepancy between what the ad describes and what the mechanic's prebuy report describes, which indicates that either the ad is "hastily put together" or intentionally misleading (neither is good). Also the idea that an engine was "rebuilt" but never logged is a huge red flag.
Were I in your situation (and I was not too long ago) I would only consider purchasing this aircraft if you could talk to the inspecting mechanic in person (preferably while seeing the aircraft), and I would ask their opinion on the condition of the airframe, engine, accessories, instruments, and avionics.
In fact that is very good advice no matter how experienced you are at purchasing aircraft: Nothing beats an expert evaluation.
To answer your question about the remaining life on an airframe and engine requires a deeper understanding of the plane you're looking to buy (again, talk to your mechanic about this - I really can't emphasize that enough).
You'd be best served by first understanding what Tach and Hobbs hours mean, and how engine time is described. Armed with that knowledge, let's forge on and figure out what times are important for this particular plane.
First you'll notice that the Tach and Hobbs times in the mechanic's report are different -- that's OK, they're probably never going to be exactly the same. You'll also notice that none of these numbers match the "Time Since Major Overhaul" for the engine -- that's also normal (TSMOH is a derived number - the current time minus the time when the last overhaul was done - so there's almost never a direct representation of it in the cockpit).
You'll also notice the Tach and Hobbs, and Total Time (airframe time) don't match up with the total time for the airframe. That's a bit of a red flag: Normally one of these is what's being recorded as "Total Time - Airframe" (TTAF), and if none of them match either the tach / hobbs meter was replaced at some point and not "run up" to match the airframe time, or there's a error in the ad as I mentioned earlier.
Your mechanic should be able to tell you which situation you're in by a careful examination of the logs, and that should also reveal how many hours are on the engine and airframe.
For the sake of argument I'm going to assume the Tach time above is the Total Time - Airframe (about 8100 hours), and we'll go with 2100 hours since overhaul on the engine because that's what the logs apparently say.
So we've established the Total Time for the airframe (about 8100 hours), and the time since the engine was last overhauled (2100 hours) -- what does that mean? How long will this plane last?
The short answer is there's no way to know just based on hours -- this is why we pay mechanics to do a thorough prebuy inspection. A 20,000 hour airframe that has had excellent maintenance could easily go another 20,000 hours, but a 2,000 hour airframe that has been abused and neglected may be ready to be scrapped.
For the engine, most GA Piston engines have a 2000 hour Time Between Overhaul (TBO) recommended by the manufacturer. Overhauling at this point isn't mandatory for most part 91 operations and well-cared-for engines can continue to operate for hundreds of hours beyond the recommended overhaul point with no problems, but generally if you're looking to buy an airplane an engine that's got 2100 hours on it will be considered "run out". Consider your mechanic's opinion of this engine carefully, and have a plan in your mind as to what you're going to do when the engine needs to be torn down.
For the airframe 10,000 hours is usually considered to be "high time" (often these have been "working airplanes" - flight school trainers, etc.) - This doesn't mean the plane is worn out, but it bears special consideration in the inspection.
Remember that nothing can ever "reset" airframe time the way overhauls or engine replacement can for an engine: The parts on that airframe have their accumulated time (and accumulated fatigue cycles) forever.
A 10,000 hour airframe can continue to give years of excellent service, with one major caveat: Life-Limited Parts. Some aircraft have components which have a maximum service life limit (for example, the wings on a Grumman AA-5 have a 12,000 hour life limit. Once reached the wings are no longer airworthy and must be replaced, and as Grumman is no longer making those planes you'd be hard pressed to find a replacement wing).
In consultation with your mechanic you should consider the time remaining on any life-limited parts as well as the overall time on the airframe and its condition in order to ascertain how many more hours the plane can realistically fly for.
Sometimes the numbers can be mind-boggling: When I was plane shopping I looked at a Cherokee 140 with 10,000 hours on the airframe. It was a flight school trainer, but somehow managed to get through its life with no major damage and had been meticulously maintained by the schools which owned it over the years. My mechanic's opinion was that it could easily fly another 20,000 hours or more if it continued to get the same kind of lavish care, which is more hours than many GA pilots will ever have in their logbooks. (Piper says the spar is good for 30,000 hours per Service Bulletin 886, after which they want it inspected).
Other times the numbers can be soberingly obvious (like Grumman AA-5 with 11,500 hours - obviously a plane in the twilight years of its service life).
So what's the bottom line?
The aircraft you're describing has mid-to-high time airframe by most standards.
The engine is "run out" based on the time since it was last overhauled.
Both could continue to give years of excellent service, or they could fall apart tomorrow: The only way to have confidence either way is to find a mechanic who knows that particular type of aircraft, hire them, and have them go over the plane and logs. Then sit down with that mechanic and go over their findings.
Your mechanic will tell you if the plane is a gem or a lemon, and if they know the aircraft type they've got a better idea what to look for than you, I, or most folks kicking around on the internet would.
