I own a '75 CB550F. Great bike. Bought it new as a high school graduation present to myself. Still ride it.
Parts are fairly readily available either through the dealer or catalogues as these motors were produced for a number of years. You might also want to look at this on line forum "forums.sohc4.net".
These bikes are becoming somewhat of a collectors item. I recently saw a mint condition '75 CB550 with 800 original miles sold by a collector in England for $5,000 US. Others in good to excellent condition are being listed for $1500 - $2500 US.
I believe the 400 fours to be the rarer of this breed while the 750's the most common. If it should come down to a choice between the 500 and the 550, get the 550.
Even at almost 35 years old, it takes very little effort for me to start it up in the spring after sitting all winter in the garage. Yes, the garage is heated somewhat, but still. These are proven, reliable and relatively easy to work on.
I'd go for it. Good Luck!
Something else you may want to consider is a late model Triumph, Ducati or Moto Guzzi. They all now make what they're calling "modern classics". Combining the classic looks of the '70's nakeds with modern technology. Likely more than $3,000 though even used.