Not sure if any of the above has answered your question.....
Retro or modern kit?
Most people on the Retrobike rides tend to wear modern stuff, with modern pedals. Some people use the retro stuff but it's mostly limited to retro jerseys which haven't really changed much in the last 20 years so you wouldn't notice much difference. There is a couple of people I've seen with retro helmets (including an Etto one which I would love!) but it isn't something I'd use, and you don't see them for sale often anyway. Same with shoes, although I would use some Axo Ponys if I could find some as they are very good.
I would recommend modern shorts/baggies/helmet/shoes for comfort and safety, with maybe a retro top to match your bike if you can find one.
Why not have a look at the retrobike.co.uk kit for sale in the shop? I have one of the jerseys and it's excellent, well worth the money. I also have the socks which are good too. Then everyone can see you like your old bikes but you are still comfortable!! I want to get some of the RB baggy shorts and maybe jacket one day too. As they are black and white they tend to match most bikes too.
John tends to do some nice packages with a bit of money off sometimes, and they are always a couple of quid cheaper when he sells them at the National rides, btu you might have to let him know in advance what sizes to bring.
Hope that helps.