By and large there are lots of bikes out there that have the potential to become future classics and of course some of them already are.

If you are interested in really old stuff such as Triton 650, Norton Model 50, Indian Chief or perhaps a 1994 Triumph Speedtwin than vintagebike.co.uk is a good starting point for you.

If you want to spend anything between £ 1500- and £ 5000- and have the storage space and the time to see your project appreciate in value, than modern japanese classics as well as some european machines are a good starting point.

Some experts recommend investing in bikes with a distinct race history,being it japanese or european but these bikes will already attract good prices now and are sometimes overrated.

Modern japanese classics date from the early 70's to early 90's and many of these bikes will appreciate in value soon. The same applies to some Apilia's BMW's , Ducati's and Triumph's from the same era. There is generally plenty of stock to choose from, parts availability is usually good, the level of engineering was already high and the bodywork was mostly of a high finish as well.
Apart from exhausts.

There are also notorious exceptions to all of this of course, and we recommend to shortlist a few candidates and to do the groundwork on them. There is loads of info out there, owners clubs, magazines such as Practical Sportbikes, MCN- Motorcyclenews and others.

Try avoiding bikes with a high mileage, even if it is in good nick it would not be a good investment.
Stay clear of hobby racers, as soon as I hear "track day" I start to glaze over.
I also find it a warning sign if a bike has had too many owners in its life- if everybody wanted to part with it very quickly there must be some reason for it.

As a rule of thumb, everything that is in good original condition with relative low mileage and unfettered with will probably gain in value at some point. If your beauty does not already have a stainless steel exhaust system it probably will need one sooner or later. Stock exhausts during these years were simply awful.

There is a well known auction site out there which is headquartered in the United States I've been told, but things can heat up a bit during the bidding process and you might end up paying more than you actually intended to. Personally I would not buy anything that I did not view beforehand.

There are also loads of magazines here in the UK with good old fashioned classic ads and true gems can be found amongst them.
Make sure you know as much as possible about the particular problems of any particular bike you are interested, it could be a dodgy shock, a leaking oil cooler, the linkage bearings, a disintegrating generator stator or the notorious regulator rectifier. Apart from the downpipes, collector box and end can.

Once identified, can you get these spares and where from?

Perhaps have a look at these:

Aprilia RS 125
BMW R 1150 GS
BMW R 100
Ducati 916 (right now everyone wants one it seems)
Honda XRV 750 Africa Twin (same applies)
Honda Hornet
Honda CBR 900 RR Fireblade
Honda XL 600 V Transalp
Kawasaki ZR 750Zephyr
Kawasaki GPZ 900 R
Suzuki DR 350
Yamaha XT 600 Tenere
Triumph Tiger 900

BMW R 27
( I owned one when I was just 18, it was in really good condition and it cost me about £ 500- then. I wish I never sold it)