Its tough to answer as the question needs to be cleaned up, qualified and quantified.
Basically, the CB750 can go 100,000+ miles. With the routine maintenance. It'll get a little rattley, but it'll be fine. 45,000 is high relative to motorcycles in general, but not to a CB750. You won't find many with that much mileage, doesn't mean they (and many other models) can't do it. It means motorcycles are toys and owners often buy them and never use them.
The problems come in from age as much as use. And what happened in all those periods of less use. Was the 45,000 put on early in life, late in life, or spread throughout?
I'd rather the miles have been put on recently as the owner would have had to deal with the issues brought on by age and sitting.
If the use was early, and its been sitting, you'll have issues with condensation in the cables, electrics, front forks, aged rubber (tires older than 6 years need to be replaced before serious use). Carburetors will give you problems, gas tank may be rusty inside, etc.
If a lot of these issues are minor, the price is decent. $1,000 would be better, tires are a negotiating point. Get the age from the DOT stamp on the tire. Go armed with DOT style warnings from the internet about using old tires on the street. Could use new steering bearings, likely rear shocks are sacked.
Its an old bike, you'll have to work on it as you go.
PS: all maintenance is NEVER correct.
