As an owner of an original condition (3 weeks ago anyway) 79 Special II, I will say if you are not in love with making an old bike purr, it might not be for you. Let's start with the known faults listed by the owner.
First of all, if you can't make it run in his driveway, it's a 500-600 dollar bike for being complete.
Carbs need cleaning. That can be anything from 2 cans of carb
cleaner and an afternoon to 100 bucks EACH for a new set of diaphragms, 50 bucks for a pair of floats, a FULL disassembly if the shaft seals are worn, and dead housings (carb bodies junk!) if they have been worn long enough to oblong the holes, and rest assured all the hoses are crapped out on those BS38's unless they have been re-worked in the past 10 years. An option (but only if you want to get rid of the stock airbox and use pod filters) is a set of carbs off an EX500. They can be made to run excellent with minimal work if you research a starting point first.
Needs a new battery. That's an invitation to a pile of problems. Could be that it really does just need a battery, but it can (and has) go like this: You throw a new battery in it and find out it's not charging and won't fire, you could be looking at 300-500 dollars to get the charging system going, and another 300 for electronic ignition.
I just finished getting mine running, and even though it charges, I have excessive voltage drop through the fuse panel and switchgear. A new fuse panel with blade type fuses is in order, and I have to take apart the ignition switch and all the handlebar switches and clean all the contacts, plus pull all the bulbs and clean the sockets and check and clean every single ground on the bike. This is a relatively weak charging system.
I installed the Pamco electronic ignition with electronic advance with a Dyna
mini coil. That's right at 300 bucks before you make new wires with 5k resistor caps, or use resistor plugs (which is what I did) but it transforms the character of an old bike. I've "known" my bike since it was on the showroom floor, and know how it ran, when it ran, and how it was stored.
Since you cannot run the motor right now, the only way you will know if the front cam chain guide is falling apart is to pull the sump plate off the bike and look for chunks of black plastic in the sump filter. You have to pull the motor to change the 53 dollar part. You may get it running and find that it smokes and needs valve stem seals. Motor comes out for that too. You may get it running and find that the clutch slips, then you're out for plates and springs. The front brakes, unless they have been well maintained, are notoriously weak, and even if they work well, need a smaller bore master cylinder and stainless steel lines for GOOD braking.
I'm not trying to shoo you away from it, just letting you know it can be kick and go, or kicking a bee's nest. Not to mention, I'm talking necessary stuff here, not even trying to make it look good, which is even more money.
One word on Mike's XS, the quality of many of their parts goes from random to junk. They have many GOOD items that they are pretty much exclusive suppliers for, but I'd advise some time searching on XS650.com before buying ANYTHING from Mike's XS, just to ensure you don't get into any of the random quality stuff. I have my bike going without buying anything from them. Their communication and return policies suck as well.
I would not pay that much for a non-runner is the bottom line. Many folks believe that a complete clean slate XS is worth that money, but if you are going to cut the thing up, complete don't mean very much, and a non-runner is a project of undetermined scale, cutting, restoring, or just making a runner.
You will not be likely to flip that to someone else as a non-runner for even 900 bucks, so if you go for it, get into it for the right money.
It's common for the ODO numbers to be off line on higher mileage bikes, so don't worry about that.
PS, these ARE great old bikes.