someone else was asking what bike for the Hebrides…agree with just about all the advice.
Don’t know about make & model. Something with low gears & decent brakes for the hills of Harris, drop bars for the headwind.
Roads are generally pretty good so you don’t need suspension or discs (but he might want them).
Buying secondhand probably is a good bet, my touring bikes are budget bitzas but the wheels are worth spending money on – good handbuilt deore/DT/Mavic kind of thing is adequate.
But I’ve seen (and toured with) folks riding real falling apart junk and they seem to do OK.
Tips? Go in May or possibly September. Riding something with 32-35mm tyres lets you explore the tracks & headland that are probably the best bits. You can just leave a loaded bike unlocked at a deserted dead end, wander off for a picnic, and it’ll still be there when you return.
Did this route in 2009, also variations on it, loved it.