OE Volvo valve guides will be fairly expensive and the test to see if the originals need to be replaced is simple - you'll learn it in the class. Hold off until then. There are plenty of B21/23/230 engines with very high mileages which have never had head or gasket troubles. Your present head and engine may outlast the rest of the car, and you'll end up putting your spare on eBay.

Buying new valve stem seals and the hushers (little rubber disks which are supposed to quieten things) would be a good idea and they're cheap. Installing them will give you some experience at disassembling the head without a lot of special tools. And these are the items your present head is more likely to need.

Storage: I'd spray it lightly with WD40 and seal it up in plastic sheeting. Store in a dry place that doesn't get big temperature swings.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)