A small desktop router may indeed be perfectly acceptable for you as they are much cheaper. They are also not meant for as heavy of work, so the aluminum moldmaking may come to be an issue. You will have to determine that balance.
One word of advice in any case; Though the routers can generally be just as accurate as the mills, if lighter duty, the spindles are a world apart. The router spindles tend to be much cheaper, much faster, and much sloppier than you need - usually by an order of magnitude on each count. While the usual hand router, a dremel-esque tool, or laminate trimmer choices all make perfectly acceptable and even optimal tools for most routing tasks, they do not work for your needs at all. You will need a much more accurate spindle with a broader array of collet choices.
This isn't a problem, it can be a perfectly workable configuration for you with a small desk router, but you need to be aware of the difference. Depending on choice of spindle, this may also remove much of the cost benefit of a router over a mill, which is another thing to take into account.
If you are feeling either really adventurous or somewhat patient, there is a new supplier of router-type machines that look to be exactly designed for your purposes. They are at
www.vivaek.com, and they are cheap and appear to be an impossibly good fit for what you need. One catch, no one has tried them yet! I have ordered some component parts from them and have been happy with the quality and price of those, but I don't think anyone here has sprung for a whole mill package yet. I liked the looks of it enough I'm confident - I'm now ordering one for myself to try out - but it's a complete crapshoot as to how good they really are until one of us actually tears one apart and sees what makes it tick.