SSL are independent of the shopping cart that you use. So basically, anyone would do the trick. If you are targetting low dollar value transactions then the basic SSL should do the trick. If however, your transactions can run up to the thousand dollar range per transaction and above then getting a higher end one which gives your customers the assurance that you are who you claim you are would be better. In the end, it is all an issue of perception.
We use Comodo for our SSL needs. They are a bit smaller than Verisign (since Verisign has not been able to buy them out) and cheaper than Verisign, but a bit more expensive then GeoTrust. Mainly you want to make sure the cert is compatible with most browsers (which most are) and support 128 bit encryption
I would suggest completessl - GoDaddy can be a pain on the SSL front. Not worth the frustration of their order process.
Pretty much any SSL is going to be fine. If you want any sort of trust seal, I would only go with a Verisign cert. It's really the only one that is recognized widely enough to even closely justify the high cost of it. Personally I would start out with a godaddy, or geocerts and switch to a Verisign if you later feel you need the benefits of the Verisign seal. As far as benefits, there definitely are some with the Verisign cert. I've tested a lot before and after we switched to it, and I think that it paid for itself within about 3 weeks. It probably completely depends on the type of product / service you are selling, and your audience, but the Verisign cert could definitely be worth it.
Zen-Cart itself doesn't really care what SSL Certificate you use. I will say that Google Checkout is known to have certain problems with Comodo certs (chained certs as opposed to single root). NameCheap has RapidSSL for $15 a year or something. Decent Seal, based on the GEOTrust root, not a bad way to go if you ask me. Also, you can make your own seal. I always like to point out the example in the upper right hand corner of CoffeeAM... CoffeeAM.com. Good Luck! Ari
Namecheap has a special running on rapidssl and geotrust SSL : http://www.namecheap.com/special-offers-discounts.asp#rapidssl13