Hi
After been in the UK racing near 5 months I will tell you, go for 13.5T open with a speedo that allows you to go easily to 13.5T blinky. On this way you can race a lot as you will be able to do 2 day nationals, clubmans and btcc for outdoors and on indoors with blinky you will have access to the ews and other series.
Regarding chassis, go for something is common in your area. In my club I am the only one racing Yokomo, but I can have parts easily so I am not very worried.
Cheers
Javier