Going for £30 in garage sale - a possible candidate for an expedition bike.
Buying a second hand mountain bike and modifiying it is the cheapest way of getting a bike suitable for adventure touring. It is possible to get a bike and upgrade it for a budget of around £300-400. This option has some distinct advantages:
The £700-£1000 you will save over an top-end expedition bike will pay for an air fare to somewhere exotic or 2-3 months living expenses on the road.
Bikes depreciate rapidly, so you can get quite alot of bike for not much cash.
There's not that much to go wrong with a bike, so it is reasonably safe to buy second hand.
If you don't already have good bike maintenance skills, the process of upgrading the bike will help develop them.
You'll end up with a pretty robust bike.
It is almost impossibly expensive to insure a bike for a long trip. So, if your £300 bike gets nicked on the road it won't be quite as traumatic as if you had spent £1,500.
One of the things that keeps you safe on tour is looking too poor to be worth mugging. So, a scruffy MTB is no bad thing.
The main disadvantages are:
MTB frame geometry is not ideal for racks and panniers. The chainstay (the distance between the pedals and the rear axle) is a little shorter than it would be on a purpose designed tourer. Hence you may find the back of your heels catching your panniers when you pedal. This can be overcome by racks which allow the rear panniers to sit a bit further back, e.g. Tubus Logo.
The frame may not have the bosses to fit the racks to. You can still fit racks, but they may not be quite as secure.
What you are looking for
You are after hard-tail, i.e. no suspension, preferably steel, older style mountain or hybrid bike from a reputable brand such as: Trek, Cannondale, Dawes, Specialised, Marin, or Giant, for less than £200. Ideally, you want a bike that has had an easy life, i.e., used for commuting rather than hammering down mountain tracks (NB 90% of MTBs sold never go off road). If you are able to check the bike out before you buy, things to look out for are:
any obvious dings or dents to the fame;
lateral play in the wheels;>
headset play (apply the front brake and try and move the bike - there should be no play);
reasonably smooth gear changes;
quality and wear of the drive train components.
Where to look
The cycling magazine classifieds, Ebay and Police auctions are all good sources of cheap bikes. If you are not going to be able to try the bike before you buy it, e.g. if it is on Ebay, you need to know the frame size you require - there is a guide at bicyclenet.co.uk.
It is important to allow for the cost of upgrading the bike in your budget.
To get your bike expedition ready you will almost certainly need to:
Replace the tyres (£50-60)
Fit front and rear racks (£120-150)
Depending on the bike's spec and condition you may need/want to do one or more of the following:
replace worn out drive train components, e.g. chainset, chain and rear cassette (~£50);
fit more suitable wheels (£100-200);
replace the saddle (£30-40)
fit bar ends (£15-20);
fit additional water bottle cages (£10)
fit new pedals - toe-clips or clipless (£20-80)
To keep costs down, shop around on the internet, use Ebay (good for cut price components, e.g. chainsets derailleurs) and don't buy the latest issue gear. Look out for end of line and clearance stocks.