Michael....you're a nice young bloke and it's refreshing to see a sixteen year old so keen on old bikes. I'd love to see you stay in the vintage movement for the long haul rather than on the way through, enroute to other activities.
BUT if you really want to race pre '75 and remain keen on old bikes, the TM 400 Suzuki is NOT the bike for you, no matter whether Dad rode a TM 125 and you're the biggest Suzuki fan on Earth. They are evil, ill concieved paralysers. As cool as they look and bearing in mind youthful thrill seeking, the TM400 will very quickly turn you off old bikes forever. If your mind is set on pre '75 and it has to be a Suzuki, be kind to your body and get a nice TM250. That way you'll at least be able to enjoy your 21st birthday without the need for a walking frame. Merry Christmas mate.