Hi, 'been working and running and riding an Enfield in Nepal for three years. I'd agree with all the above, but a few points to add.
With a bike simply forget importing (due to cost) or buying secondhand (due to appalling mechanical skills). If you're coming and wanting to ride back, or on doing any big ride, plan to be buying a new bike. Yes tax is high but my brand new disc brake 5 speed Bullet cost me less than 1,600GBP (same in Euro these days !). You could ride it home sell it, and just about cover your costs.
As said previously on this string though, that'll take the best part of a month 'cos once you're registered you'll have to get you Carnet sent out from what ever country you're going to arrange it with (your own). In UK this simply isn't a problem, that much I know. Anyway there are worse and more expensive places to stay for a month than Nepal
The last thing is a word of advise on Enfields. There are a lot of foriegn riders on Enfields in Nepal who simply don't ride them and dont know the back wheel from the front. Yes they get ripped off, but they deserve to when they go shouting into workshops wanting this that and the other tomorrow so they can do their annual 5km ride (a lot also dont seem to know about the tax issues which mean spares parts are much more than in India - though still way cheaper than Europe, etc).
This is Nepal so come with a view to sit around a lot and drink tea..things happen when they happen in regards to paperwork, bike repairs, etc. I previously worked about 200kms from 'Himalayan Enfielders' and on three occassiosn they called me out to deal with 'problem foriengers'. i.e shouting, ranting, raving idiots, who would only accept mechnaical adivse, etc, from a white person..very sad, but act like a prat here an you'll be treated like one and get ripped off (and will deserve it).
'Enfielders' is indeed not what is was, but Imtaz IS OK as a mechanic (ask for him specifically), but No mechnics here have 'mechnianical sympathy' so they're OK to bolt things together, but never let them near an engine rebuild or anything (though Raju in Pokhara is OK) !
one last thing. I'm just about to set off back home - India, Pakistan, Iran, etc (to UK) but may have an Iran visa issue due to previous work experience there (I work for NGOs, and they have 'issues' with anyone they know was involved in the Bam earthquake response). Anyway, if I fail to get my Iran visa (I'll know in three weeks) my bike
could be up for grabs. This a 30K (no problems, I maintian it myself and have changed the oil every 1000kms - crazy I know, but I'm like that with engines !) Enfield Bullet, with 30k KMs done over three years, Givi panniers with re-inforced frames, 18 (rather than 14) lires tank, UK Dunlop K181 tyres, gel seat, full spare ignition system and on-the-road spares, etc, all set up and ready to go (even has a carnet, but obvously in my name, but all reg papers, etc are already lodged wit the RAC in the UK so issuing a Carnet in your name would take them only minutes - plus your cheque

of course). Blue book is in my name and takes only a letter from me to tranfer it to you. Anyway keep your eyes on the site if your interested, as if its up for sale it'll be in about three weeks. If you don't see it I got my visa and 'am on my way
Cheers
Pete
PS. If anyone is serious in a new bike put up a post as I'm going down the importers on monday and can ask new prices for wat ever they currently have in stok (in the past they have run out of new bikes for MONTHS so be warned. Don't turn up and expect there to be rows and rows to choose from !)