welcome.

regaring rust. i don't think they rust any more than other cars of simular age. as most people are keen cosworth fans they tend to be well looked after.
my opinion is. before winter give it a good power wash off underneath and waxoil,then do the same in spring.

servicing. make sure you take it to a decent well known garage. it should certainly be cheaper than a modern car.

security. you can never have enough security as far as a cosworth is concerned.

The only problem i can see is parts if you bump it. most are available second hand, but not new.