Unfortunately racing in any class at can be expensive, its quite easy to get through £200 of tyres and wheels at a national in 1/10th when the tyres are only good for 5 minutes!

Milliput is pretty good but it needs to be applied well. t has to be done with water as it helps it flow and is easier to smooth and as with any bonding the surface has to be well prepared.
To upgrade fully to an X11 would be more than £100 unfortunatley.
you'd need:-
X brace upgrade kit £75
http://www.the-pred.co.uk/zencart/index.php?main_page=product_info&products_id=85Pivot blocks x 4
about £80 total
Lipo Chassis £41.89
Wisbones upper and lower front and rear
about £40
Hing pins
about £15
Big bore shocks
http://www.the-pred.co.uk/zencart/index.php?main_page=product_info&products_id=69£70 (for 4)
There may be a few other bits but i think thats the main parts.
So the total would be at least £320 (i've probably missed a few bits)
It would probably be better to sell the car and put it towards an X1108 if thats what your after... but there is nothing wrong with the X10. You could upgrade the shocks and get the X brace kit which is much stronger than the X10 setup.
Hope that gives you an idea!
Kev