I wouldn't buy a new system... the markets are flooded with 2-3 year old quality systems with 500, 1200, 2500 hours. Probably from guys like you who thought having a new system was the way to go. Then money got tight for whatever reason, and now they are out of business.. not that I'm saying that will happen to you, but only as a word of caution.
Buy the used one for 15k, cancel your RX-20 order. If you barter hard enough you can probably get it for a few under 15k. Use the 12K left over to upgrade your system to 2.5 in plumbing, hoses and greenhorn wand. Take what's left over and split it in half. Put half into marketing. Monthly postcard direct mail, join a BNI, Chamber, build a website, discount coupons to hand out, etc.. Put the other half in the bank. Each month buy a $500 or $1000 6 month CD.. after a while you'll have a rotating monthly CD coming due that you can use for training, equipment purchases, marketing, etc.. If you don't need it, it will go right back in for another 6 months and continue earning interest. At least you'll be getting some interest while the money isn't being used and it's a little harder to get at to prevent any dumb impulse purchases.
You NEED a business plan!! Speaking from experience here.. I didn't have one and I wish I did!!! Talk to Service Corp of Retired Executives or SCORE. They have workshops on all things related to business. They can help you build a business and marketing plan and give you some really usefull information and insight into things you probably never thought of.
Just because you've got 27k available doesn't mean you have to spend it all on equipment. Save as much as you can and purchase things as the growth of business dictates. Your van insurance, liablity insurance, phone, cell, and other fixed expenses will be there every month whether your busy or not. So what if you are debt free with no van or machine payments but are cash poor and can't advertise because you don't have any money left over and the phone isn't ringing... you'll be pissed off for sure.
Good luck