The advice you received is accurate, but some subs are more capable of running in free-air/infinite baffle locations than others - like this one :
I don't intend to push the sub, I don't want to win any boombox contests, just wanted some musical bass. Also keep in mind that the storage compartment on the Spider is quite a small volume so its not exactly a free air installation.
Here's the sub I fitted : http://www.alpine-europe.com/index.php?id=swe-843e
it's working great in the luggage compartment and it's a very nice fit mounted in the original location between the front seats. It was a bit of a risk as to whether it would work well or not but I'm just lucky I guess
As for the £100 budget:
1. The Pioneer single-CD head unit was pulled from my wifes old car (the car that the spider replaced).
2. The Spider's previous owner had bought and fitted (badly) the AudioBahn component speakers - they sounded awful but this was dramatically improved by wiring correctly and mounting solidly into the door. The cost for this was some speaker wire and two peices of MDF
3. The JBL amp (for the sub - 120w RMS bridged) was bought new on special offer from a local car audio dealer @ £40 - an absolute bargain.
4. The Alpine sub cost £40 on special order from the same audio dealer.
5. Spent £20 on speaker cable, MDF, audio connectors, cable for amp wiring, velcro, etc etc etc
That's it ! £100 ! and a lot of hard work too but hearing the result is definately worth it.