I bought my 1991 k75s for $500 earlier this summer from a seller that had a similar story to your seller. He'd ridden the bike for a couple years, had the bike serviced regularly, etc. He and his wife had a child and he quit riding. Bike sat in his garage for a couple years then he decided to sell it.
I'm an instructor in a motorcycle mechanics program so I have plenty of time to work on the bike. I drained the fuel, replaced the fuel filter and gas and verified the oil was good enough to crank it. Hooked up to a jump box and the bike fired right up. I ran it on the dyno to make sure it had no transmission issues and shut it down. At that point I changed all fluids, put on new tires, lubed the splines, replaced the coolant, radiator cap, fork oil, etc. I've been riding the bike around 100 miles a day for the past 3 weeks, took it on a nice day trip to the North Carolina mountains this past weekend, and when the summer semester is over I plan on taking a couple week road trip to where ever I end up.
All that being said, I'd pay $750 for the bike you're looking at with no hesitation. I have the time and ability to fix it. If you don't have the time and ability, as others have said, it could turn into a money pit. The good thing is, for that price, you should be able to unload some parts to get your money back and then some. If your brother can go check it out have him check the obvious things, make sure it doesn't roll when in gear, make sure it shifts, take a look at the wiring, check for rust, etc. Consider your wants/needs/abilities, it looks like it'd be a great bike with a little time and effort.
My $500 brick-
