Yeah, a downpayment of 10% seems reasonable. But, if they are being pushy, then you need to tell them to slow their roll.
And, if you are going to put $ up front, then it is perfectly reasonable to ask them for references. Meaning, ask them to get you in touch with past customers so you can speak to them about the company and perhaps even pay a visit to see past work first hand. If they refuse to give references, then that is a sign to move along. Also, checking out the reputation with the BBB is a good idea.
Now, a very important item to consider is that whatever company you go with will almost certainly be a dealer for particular brands. That means they absolutely will not sell or install a brand that they aren't a dealer for. So, that right there could be a deal-breaker. And, of course they will try to tell you why brand A (that they sell) is sooooo much better than brand B (that they don't sell).