"They have asked for a valuation and if we would want to become full time employees."
As another poster said, it's a sales process. You should appear delighted at the prospect of working there. You can always say no later, but they should be wowed by your enthusiasm.
Regarding valuation, make them offer a #. A good way might be, "for a company at our stage, a formal valuation is pretty hit and miss. We kinda feel like the market should set our value. You guys certainly have more experience buying companies than we do selling them-- and we'd like you to make an offer." If they push you could always make noises about the best way being to see what a couple of other buyers might pay. ;-)
I'd avoid a lawyer for a bit-- they aren't going to help much with valuation.
The variables in the deal we did was:
1) stock options (part of employment agreement), vesting over 4 years
2) cash
3) cash signing bonus (just a way for them to put it in a different budget line)
4) relocation allowance
5) salary
Treat the employment stuff as separate. My buyer laid off 40% of the company in 6 months... I made the cut, but as easily couldn't have and would've lost my options and salary. Assume that'll happen to you (which means focus on cash, signing bonus, and relo variables).