My bike is about 14 years old. I was lucky, the previous owner took very good care of it. So far, nothing unusual except for routine maintenance, which I do myself.
One of the first things I got for my bike was a shop manual. Cost me about $25 on EBAY. The best investment I made so far on my ride. A shop manual will pay for itself the first time you use it. The shop manual will also let you know if you are getting in over your head. Even if you do not do the maintenance yourself, if you read the manual, you will understand exactly what the motorcycle shop is doing.
As far as the routine maintenance goes, I did a little research on the Internet for my parts. I was able to cross reference my oil filter to a Wix filter. These are just about the best oil filters on the market. If you have ever seen the store display with a cutaway of a Wix filter and a conventional filter, you will understand why. You can get these from any major auto parts store that sells Wix filters. They may need to special order one. They even cost less than the cheap made in China filter you will get at your motorcycle dealer.
I was also able to cross reference my spark plugs to some Iridium plugs that I special ordered from my favorite automotive parts store. Again, the Iridium plugs from the automotive parts store were less than the conventional plugs from the motorcycle dealer.
I also found a very good website for motorcycle parts. They can get you parts for some surprisingly old bikes. See the link below. However, keep in mind that motorcycle parts can be expensive. This applies to both old and new bikes.