Change the belts immediately!! Then use it, put at least 2000 miles on it before breaking it so you know it's not using oil and doesn't have any head gasket issues etc etc. There's little worse than putting a duff engine from a known car that you'll be using to get an age related plate after IVA. A rebuild at a later date will be more expensive. As a consequence you might be tempted to just get another engine which, at registration, will probably mean th dreaded "Q" plate which might affect the value of the finished article.

End of the day it's your decision and your build though. You don't have to listen to anyone!

Guy