You have to take the car to a certified AC tech. to get the refrigerant evacuated.
Technically not correct.
As a mechanic doing work for hire, you have to reclaim/recycle all referent from an AC system. As a private person you are free to vent it to your heart's content (as long as your neighborhood "greenies" don't catch you
You can vent the referent R-134a from the system. Then when you are done with your repairs, you can vacuum down the system (you can find AC vacuum pumps easily for $100-150), and refill it with R-134a. You can use several individual "1 lb" cans (around $5 a lb) or do what I did - buy a 30 lb tank (around $2 a lb). You need 3 to 7 lbs depending on the car (I don't know what the Elise takes).
I bought all the AC repair equipment (vacuum pump, gauges, leak detector sensor, weight scale) a while back when I had a leak in one of our cars. Buying all the equipment (and the new sensor that was leaking) was less than the price quoted to me at a local shop (and he wasn't even going to replace the part that was actually leaking). Other shops gave similar estimates.
I even did the on-line test to get certified as an AC tech so that I can legally buy R-12 "freon" (for pre-94 or so cars).
But Michael is correct, the proper way (and environmentally correct) to do things is to take it to a shop, have them vacuum out the existing R-134a and recycle it. Then take it back and have them re-charge it when everything is done.
Oh, one thing. If you open the system (i.e. remove the condenser), then ensure that you cap ALL the openings to prevent the system from absorbing moisture from the air. Other wise, you will have to replace the drier/accumulator as part of the repair (it's job is to absorb the moisture that is trapped in the AC system).