Regarding baby signing, ime 7 months isn't early to go along to a class. We started when DC was 4.5 months - and there were much younger children there.
The classes were a good social thing for DC and for me. Agree with silverfrog that the classes are lots of nursery rhymes and games with signs. The classes are designed to be stimulation for the children, and an opportunity for you to learn signs you can teach at home.
I didn't have a DVD, but it could be useful. Be aware that there are at least 2 types of baby signing: BSL-based and Makaton-based. Something Special on cBeebies is a Makaton programme.
Young babies don't learn much from seeing a sign a few times during a class, so don't worry about your child picking up / not picking up signs directly from the class. Babies learn though lots of repetition anyway so you need to sign with your child at home.
At first, I chose just a couple of signs to do at home - milk and nappy - as I felt it would be useful if DC could let us know about these. DC did start to sign these two things about 6 weeks later, but DP & I had signed several times for every single change and feed, and we kept it up after DC started to sign. Other signs were learned from us signing along to songs and when speaking.
Some mums were a bit disappointed that their child wasn't signing quickly or liked signs like bird or tree, rather than food, milk, nappy, etc. A lot depends on the child - some never sign - though most of the mums complaining readily admitted they didn't sign at home at all. However, many of these same mums are still going to the classes because their DC enjoy it, and the mums have made good friends.
My DC did pick up some more signs, but having learned to talk, doesn't use them much now.
Sorry this is epic...I just felt that there's no particular need to wait to sign and it might be nice to start something specific to help with communication because you are having to wait and see for so many things just now. Please don't be disappointed if your child turns out to be one who doesn't sign - some children never do - whether NT or not.