So buy it for £350, spend £100 and then sell for £450. So a not for profit project?
Someone where I work has one of about the same age. They are fabulous bikes and will, normally, only be beaten by age. A very good, versatile bike. I don't think you could spend as little as £100 unless you bodge the tyre and that would not be good. I would say buy it and spend if you want to keep it or leave it and find someone who would like to buy it and keep it to get them mobile again or a project. Try and keep it away from the breakers, it would be such a shame