My last Blackbird was a paralell, covered 150,00 trouble free miles and is still going strong in the hands of its current owner.
Like any bike, if it is looked after, they will go on forever. Although at the time I was in the trade so I got parts at trade price, I cannot recall one occasion where I had a major outlay. Before that I had an imported GSXR1100WP which I did 174K on, and before that two CBR1000's which covered 172,000 and 135,000 miles respectively. The 172,000 mile bike is still going strong with over 300K on the clock.
I also sold a few paralell Birds to customers and they are still fine. It all depends on who you buy from and what there aftersales are like.
Avoid German, French and Swiss originating bikes as BHP varies between 35 and 100BHP as a maximum, if possible look at the Italian sourced bikes as they are the full Monty and they come with the headlight already pointing in the right direction.
To be fair, the diference in price now between a UK and import is so little, you may be better of sticking with a UK bike if you want peace of mind, but don't be put off a bike simply because it is an import.
MPH speedo is ?90 from David Silver and dead easy to fit.