A VW engine holds a little more than 2 1/2 qts. of oil, so start with that. Does it have a bolted on sump on the bottom (a pic would help)? That will hold 1 1/2, 2 or 3 1/2 qts., depending on which one it is. A full flow filter, bolted in the left rear fenderwell with about 2 1/2 feet of oil lines will add another 1/2q. +, and the front mounted cooler has got to hold 1 or 1 1/2qts., so you're looking at 9-11 quarts of oil. Drain the oil from the sump and take the filter off and see how much oil you end up with; that will tell you if you have to open an oil cooler line somewhere. I prefer lighter oil (10w-30) as it lets the stock cooler function properly, the engine isn't trying to push around tar when it's cold and still develops enough pressure (10 lbs/1,000rpm) when warm. If with 10w-30 you find you don't have enough oil pressure then go for something a little thicker.
You don't have to bother flushing out the crankase or lines; with a filter there's really no need. Ideally, let the thing sit overnight; in a stock engine 4 more ounces of oil will drain out, so with your set up there will probably be substantially more. Though, with horizontal lines that long, air pressure may have to be added to drain the cooler and lines completely. Do the lines to and from the cooler go through a thermostat? It will look like one of these; #1 will be in the lines and #2 will be between the filter mount and filter-