James,

I'm sure you're going to have to evacuate the system before replacing the hoses and compressor. I'm torn on my system right now because I just decided to let the A/C shop install the compressor and I have a pound of Freon sitting there that I'm going to have to have evacuated before proceeding further.
I would definitely take the route of installing what parts I knew needed to be replaced (plus maybe a few small ones that you're not sure about), then taking it to the shop. Evacuating and paying for Freon that you end up having to evacuate gets very expensive. But I kick myself every time I get lazy and let the shop install a part I could install myself. I see the bill and want to brain myself.
I'm trying to use the shops, especially A/C shops, for things that I can't do myself. Buying and bolting on a compressor is fairly straightforward. You'll get hit with enough when they purge and refill the system.

Plus, if I'm not mistaken, the shop is not allowed to fill a leaky system with Freon, so your "fill it as it leaks" scenario is no longer available (thank you, EPA!).

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.