use an attorney in real estate purchase?

I'd certainly recommend using an attorney for a property purchase. I've bought/sold several properties in the 25 years we've been here, and always felt much better about the transaction because of doing so.
Property ownership and rights are quite complicated in Mexico and it's good to have someone who has a working knowledge of the ins and outs of this sort of thing. With that said, not just any attorney is a good thing --- you need the right attorney.