I've cut valve seats using manual equipment before, it's not too difficult. However, I wouldn't recommend doing it yourself unless you have a bad head to practice on first. If you mess it up the first time, it can be quite difficult to get the cuts correct afterwards and may even require replacing the seats (which is no fun) if the valve sits too low in the seat. If you do decide to do it yourself, just make very small cuts and check the contact band between the seat and valve after every cut.
BTW, the Neway cutters are the exact ones that I used. They are a very well built tool, and will easily outlive you if treated properly.