yeah, you can use the old chain, though replacing it would be like $15 and you would get a lot better performance and not wear your freewheel and chainring as fast. Also, with the old chain you will need to know how to put a chain back together without creating a tight link, if you buy a new SRAM PC-58 or something like that it will come with a master link and all you will need to do is drive out a pin, which is harder to mess up. You will need to have the shop remove your current rear gears with a special tool, or you can buy it for $10 or so, though you'll need a wrench to turn it with and if this is your only bike you probably won't use it again.

You will probably have to do some redishing and respacing on the rear wheel to get the chainline right. These will be axle spacers, not hub spacers or bottom bracket spacers. If you only need to move the chainline out in the rear 3-4mm or less, you might be able to get away with bottom bracket spacers since freewheels don't need as many threads as they actually have. How much redishing and respacing you need is hard to tell without looking at the bike in person.

And yeah, I might be interested in the old RD, though you might want to hold onto it in case you change your mind. PM me with what it says on it if you really want to sell it.