Point about stressing components during the twilight of car's life is good.
This is what I've done on my 100,00 miler 184bhp 330D Touring with 5 speed steptronic:
(1) K&N air filter
(2) Exhaust Gas Recycling valve bypassed with a unit which took about half an hour to fit.
(Available on ebay - see this: http://cgi.ebay.co.uk/BMW-EGR-Diesel-Bypass-Valve-
... Old standard one was very mucky but retained for spare. Have a look at this link: http://www.bmwland.co.uk/talker/viewtopic.php?t=83
(3) Sprintbooster bought secondhand (see review: http://www.bimmerfest.com/forums/showthread.php?t=
... am really very pleased with this because it cuts throttle lag. I found it tricky to get the clip on my car to release the throttle pedal. On a warm day the plastic is more flexible.
(4) Tuning Box CR51 bought secondhand (see: http://www.pistonheads.com/gassing/topic.asp?f=72&
Been through France & Italy twice. Up through the Alps it was a hoot. The car is used in London for shopping on regular basis. Fuel consumption 35mpg, although computer can show 45-50 ish. Performance is just a joy. Especially good is the reduction in throttle lag from the Sprintbooster
(a)Total cost around £300, although I can't actually rember what I gave for each bit;
(c) total fitting time: say, hour or two by self, with no training or engineering background.
(d) increased enjoyment - absolutely priceless. Subjective assessment of acceleration is that it feels a bit like takeoff at Gatwick. It makes my wife yelp, my daughter shriek and my son join me in Jeremy Clarkson impressions.
I find PistonHeads and BMWLand especially helpful. I regard my budget mods as a good investment. It is FAR, far cheaper than getting the car remapped. I would guess that a good re-map would be superior, but would be a miserable 2nd place in terms of cost on an old car. Better off getting the next car up the range.
Personally, I'd love a 335D.