Excellent old-bike project management advice!
Also, if it is the oil pump, there are three o-rings that are replaceable under the pump, but the pump cover has one that is not listed separately as available from Honda. This, I find, is usually the one that leaks. Honda would prefer for you to purchase a new oil pump for upwards of $180 (if still available?) to resolve this leak. However, I prefered to pay less than $2 for an o-ring at my local hardware store, remove those 3 philips-head screws, and replace it myself.
There is another list member, Kevin Hunter I believe, who has the exact spec o-ring available, or knows where to get it. If you find him he will probably mail you one for much cheaper than you'll find it elsewhere.
Unfortunately, even if you know the exact size, it is nearly impossible to order just one from an o-ring supplier. They usually have a minimum order of $30 or something, but each o-ring costs about $0.20 or so. So, it makes sense to get a couple from Kevin and just be done with it.
BTW, you mentioned that you want a commuter bike. Generally 32 year old bikes need some work to get them back to the point of everyday reliability. You might want to leave some extra time in the morning for some inevitable failures in the first few weeks/months of riding this bike. These bikes are generally very reliable, but they take some getting used to, lots of regular maintenance (like valve adjustments, chain lube, etc.), and a 32 year old bike will probably need a significant amount of rehab. Expect to have some misc. failures, and don't get discouraged. If you replace all of the common failure stuff (throttle, clutch and choke cables, chain ------ DON'T EVEN THINK OF USING AN O-RING CHAIN ON A CB550 or a CB500 -- brake pads, caliper cleanup, brake fluid, oil & filter, air filter, tires, fork seals, etc.) then you'll have a pretty reliable bike on your hands, but it might take a few months.
Congratulations,
Ed
Welcome to old Hondas. You negotiated down half of the repairs and if you're inclined to learning your bike, you can probably do them with less cash spent than that $300. I'd make a list of the repairs and do them one or two at a time so you don't end up totally disassembling it and keep it rideable most of the time.