Which SSL certificate should I buy?

As you've probably seen, SSL certs come in a variety of types and flavours. For your purposes, a relatively standard SSL certificate would suffice ; you don't need an http://en.wikipedia.org/wiki/Extended_Validation_Certificate certificate, unless you think there is benefit to the little green bar and higher level of trust from users. EV certificates are typically much more expensive and require a longer turnaround time - you will likely need to supply documentation planning you/your business are legit. My only recommendation would be to get a wildcard certificate ; this covers both yourdomain.com, but also any subdomains you attach to it - which will help if you decide to use login.yourdomain.com, or mail.yourdomain.com, etc. I've purchased certificates from http://en.gandi.net/ssl and http://ssl.comodo.com/comodo-ssl.php?key5sk1=b7400439ddaec9eb615034acf4c2cacd7f6f8d93&key5sk2=2128&key5sk3=1375551363000&key5sk4=2128&key5sk5=1375551372000&key6sk1=&key6sk2=CH290154732&key6sk3=9&key6sk4=en-us&key6sk5=GB&key6sk6=0&key6sk7=Google&key6sk8=118800&key6sk9=1280800&key6sk10=true&key6sk11=19e338537254c42a5406b26f46079c1c04e7b95a&key6sk12=2034&key7sk1=2&key1sk1=ors&key1sk2=Google. Gandi were straightforward (I also use them for domains and some hosting), Comodo were fine but a bit more difficult to deal with.

Safe from what?

If it's not an e-commerce site I don't know why you would need a certificate at all.

SSL certs are sold like mattresses: a flurry of different names to describe very similar things. The biggest differences are root-cert browser support, as aychedee says, and the amount of verification that's done by the issuer when applying. Unless you think you'll need a wildcard for multiple subdomains, I'd go with a cheapo <$10 domain-verified cert from Namecheap's http://www.namecheap.com/ssl-certificates/. Comodo definitely has a few more hoops to leap through. You'll also need to check with your hosting provider on whether you can get a separate IP for SSL, or whether it uses SNI for multiple SSL sites on the same IP, which is http://en.wikipedia.org/wiki/Server_Name_Indication#Browsers_with_support_for_TLS_server_name_indication.5B6.5D by most modern browser/OS combos, but not by any browser using Windows XP.

I really like http://www.digicert.com/. They have great support.

http://www.rapidssl.com/ is the cheapest source that I know of that has worked on every device I have tested as a client. Every lesser one I have tried or seen in use doesn't work with one or more of the test devices I can scrounge up from coworkers and friends whenever I reconsider SSL certs (which has been yearly for the past five years).

I buy from CheapSSLs.com but I think they are the same company as NameCheap mentioned above. I don't think you need a sophisticated SSL unless you are hosting very sensitive information. As long as people see https, that's all they generally care about. The only thing to consider would indeed be whether you need wildcard.