I'm hoping since the drill would be used for a one-off of, like, 9 grooves, I probably wouldn't stuff the drill up too much :)
What with the quality of dremel attachments these days, I'm very skeptical about the quality of their other router attachments. I'd hate to buy one and find it's a bit crappy. However, the possibility of just hiring a proper router and using a straight line guide is looking like a better proposition.
And what quality do you think a hired drill press would be?
Almost certainly, someone before you would have used it as a router and stuffed the bearings, alignment, etc.
I'd say that for a one-off job, your best bet is to get a machine shop to do it for you. Hiring a drill press is likely to be expensive (as is hiring any other type of tool). Buy a cheap one, and sell it again when done with, and I reckon it'll be cheaper.