When you buy a set of lowering springs for a T5 the manufacturer (H+R on my T32) will offer a range of springs to match the load of the van, you just need to choose the correct load and the numbers are written on the plate under the bonnet so you have to do this process for any T5.
If you want to lower your van more than 40-60mm (40 in my case, it gives me plenty of ground clearance for getting onto campsites/fields etc) the you replace the front suspension struts completely with what we call "coilover" suspension units. Many companies supply these and you can get them really cheap for the T26, 28 and 30 models. The T32 models have a different front strut which is beefier than the one used on the lesser models and only a couple of suppliers sell them and they cost £1300 for the whole set. You occasionally find that T30s are fitted with the stronger T32 struts.
As far as wheels and tyres go, you need to ensure you get >103 rated tyres for the T32. This means I run a subtly different tyre than your average van, they cost about £20 a corner more. I use the old Sportline wheels as they're cheap and easy to source, even though the old Sportline was based on a T30 the wheels are massively overspecced so no problems. If you get a new T32 with the 180PS engine then you need the new Sportline wheels to clear the bigger brakes - the new Sportline comes as a T32 by the way.
The only old shape T32s with big brakes (and therefore 17" steel wheels) are the 174PS models, my 130 came with 16" wheels which look the same as all the other steel wheels but are actually a stronger version.
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