The diesel box is no stronger than any other box there all the same its just the ratios that are differant dont fall into that trap.
Ive bought 3i boxes for as little as a tenner before its just a case of trawling ebay for cars local to you that are being broken and makeing people an offer personally i wouldnt pay more than £50 for a second hand box its not like your going to be running 300bhp out of your 1.3 so the chances of you blowing the bix up are pretty slim.

mark.