[Bobiam]

First.....where did the coolant go in the first place??? If that much was missing when you bought it, you had better wait a while to determine if you have a leak problem needing repair before doing a coolant change. Maybe not just a cap problem. Be sure.

Temporarily, I don't think the mixing of coolants will be a problem.

Yes, try to do the coolant change yourself.......a dealer will cost more than you paid for the car. There are good permanent substitues for the original coolant out there that will not cost as much. I can look mine up if you need a suggestion. Don't try to do the job with premix. Have distilled water on hand and do one of two distilled water flushes.
To do this right takes a few days if you do it right. Refill with distilled water, run to hot, cool down and drain, etc.

You will eventually need a decent aftermarket shop in your area. Join the Porsche Club of America in your area (WELL worth it) and get tips from local guys there. These forums are priceless, but you need local advice too.

Best of luck......