Everyone here has posted great information for you. Your job is to absorb it, massage it, and formulate a business plan which is acceptable to you.
I purchased a SS 6 bay wash a little over a year ago, it was my 1st biz venture and my family and I have absolutely 0 regrets. I'd do it again in a hearbeat! However, I just passed my 1 year anniv with it, so the novelty probably hasn't worn off yet!
What should you pay for the wash? How much is it worth to you? You have the seller's tax records, now you should have an appraisal done. Then put together your business plan, which will contain your ideas, how you plan to put those ideas to work, revenue and expense projections. Then sit down with your banker and discuss your options. Find out what the bank would be willing to loan you. Now you're armed with everything you need from a financial perspective and you can negotiate with the seller. Talk with him, shoot him a price, walk away and let him think about it. If he wants to sell, he'll call you back.
There are a lot of other things you need to think about. If you're doing this on the side and have a full time job, then prepare for the time commitment. Customer service is key in this industry, and you have to provide it. Keep the equipment running, clean lots, empty trash cans, clean bays, etc. every day. Make sure you give yourself the time to make repairs while you are there. Spend the weekends at your wash with your customers. People buy from people they like, and if your customers like you, they will return because they feel good about giving you their money. Make it their wash, not yours, you just maintain it for them. On those busy days, make sure you can skip out of work and stay at the wash. Those are the times when something breaks, right everyone?!
Repairs - you'll have plenty of them. Make sure you know about electrical wiring, plumbing, troubleshooting, etc.. If you're handy, you'll be fine. If you're not, it'll cost you a lot to hire the service work. Your local hardware stores will carry the majority of the things you'll need for repairs.
In summary - The car wash business is an absolute roller coaster. Some days you'll have a lot of quarters, dollar bills, etc.. Other days you won't see a single one! Have fun and make the decision that's right for you and your family. Just ask yourself...Why do I want to do this? What is my goal?
Best of luck!