Some kind of laser printer will be best if your volumes are high. B&W is cheaper and faster if you're happy with that, but these days colour laser printers can also be startlingly cheap (make sure you get a big set of toner cartridges with a new printer if the printer is colour, refills typically cost about £250). We use a Konica Minolta and get excellent results. Others swear by HP.
Most of our users print the plain paper payslips. These look very smart on good quality paper and you have control over colour and typestyle as well as being able to put company logoes on them.
But some users, particularly bureaus, like to use stationery, which you can buy from here: http://www.12pay.co.uk/CommonQuestion...
The RS34 style on that link is very good if you're sending the payslips through th post.