Thats my DT400 with the white tank (from a 250). 400's do come up from time to time, i have seen only 2 in the past year that were in decent nick. These bikes are now as rare as hen's teeth.
I'm currently busy with a US spec '73 dt250, and once i've done that i will begin work on the 400. I bought it (the 400) off ebay for £500, and have probably spent as much again on parts. Some bits are non existent or hard to find, so get a complete bike if you can.
I'll expect to get the bike on the road for about £2500 i should think.
Also join the VJMC and get the stafford shows, something always there!!
good luck
Chris