All good (so far)
Hi Yak Sak,
Thanks very much for getting back to me, sorry it took me ages to reply. Thanks also to everyone who has chipped in with advice on this post! I will give a wrapup of our experience to date to help out anyone else looking to do the same thing.
We finally went with two new Honda NX4 Falcons from Honda Desert Racing in Miraflores. Took us a week or so to decide on which bikes and from where and to negotiate a decent price, then 2 weeks for all our paperwork to come through (tarjeta - title of ownership and our number plates), but besides a 2 day delay for the second bike due to an error in spelling one of our names, the whole process went smoothly. We also went beforehand and got our "permiso especial para firmer contracto" as we heard this COULD be required. The bike shop said we wouldn't need it, but we preferred to spend the extra $us50 instead of waiting an extra 2 weeks if our applications were refused.
Once we had our tarjeta, we went straight to the La Positiva office in Miraflores (near the brazilian embassy) and managed to buy SOAT for only 1 month directly from them. Nice little saving considering we only need a month now, and maybe a month when we come back to Peru (much cheaper than paying the whole year). Most people we talked to said we could only buy for a year, but I think it's just a hassle for them, so they don't like to do it. It IS possible though, as we found out, it just helps to be really nice and speak enough Spanish for them to want to help out.
We are in Arequipa now, bikes are running awesome and no problems with the altitude so far (had them up to 4780m with no issues at all). We will attempt the crossing into Bolivia in a week or so, hopefully that will be easy, but that story of 300 bucks to leave was via this border, so fingers crossed. We checked with iPeru already and they told us we should not have to pay either. We did tell them we would be returning with the bikes in 5 months or so, so maybe that's different to the other guy.
Ps, enduro style bikes and a really good gps have proved to be the best choice for us, so many cool roads (and so easy to get lost)
Ride safe everyone, see you on the road!