>1) Anyone see any issues with asking a potential client for a retainer? If so, how much?

No problem. I've done it.

>
>2) Any problem with putting in a kill switch? When they pay, you give them an access key. If they
>don't enter the access key within days, software stops working.

Yes. The data is their's. The program is yours. If you prohibit them from access to the data, then you could be in trouble yourself. I build-in up front, in the contract, the issue of copyrights. I own the code until they make the final payment. At that point, I sign over the rights to them. I make it clear that no payment equates to copyright infringement and will be enforced to the letter of the law (penalties include fines and/or jail time).

Before I did that, I had to sue in court to get payment. The customer never showed and I won a default judgement and took out a lien on the guy's house.