It could be a starting point, just to see what you have - if nothing else you'll be able to compare a before and after.
The first few hp are the easiest. I think a decent tubular manifold is a must; you could cobble the Janspeed to it but depending on budget you might want to replace the whole...
MPH = (rpm x 60 x pi x tyre dia) / overall gear ratio x 63360
note tyre diameter needs to be in inches.
I ran this calc for my Toyota and used rpm at 1k intervals and comparing to different diff ratio's (I'd already decided on tyre size based on what suited better visually).
If you are...
Jon - you need to calculate this sort of stuff, don't just guess. Do you honestly think by fitting a slightly lower profile will account for changes in diff ratio ????
The tyre will look too small, you need 50 profile....... and fit a 3.9 FFS.:D
It wasn't worth the £1500 to begin with, so I expect the £3k seller will lose money on it.
When I was breaking cars, I'd be buying this for £200 max, sometimes just taking them away FOC.