I have the porter cable 890 series combo kit, and it had done everything I need it to. I started a long time ago with just a 693 plunge router, and it did just fine. The only thing the "combo" kit has been good for is leaving the fixed base mounted in the router table and keeping the plunge base out for above the table work.
Combo kits aren't a necessity for what you want to do, but more of a luxury. You definitely would be better served with a plunge style router, for the circle cutting and the o-ring groove work. You can do pretty much everything you need a fixed base router for with a plunge router, but some guys prefer a fixed base for flush trimming operations and the like.
If price is an option, you could get the old standby Hitachi M12v plunge router ($160) at amazon, cheaper refurbished on Ebay, and that would do pretty much everything you could want it to do.
If you are really strapped for your budget, or aren't sure you are going to stick with woodworking, you could then step down to a Ryobi 2hp plunge router for around $100 at Home depot. This is a decent starter router, and would make a great second router later on. (trust me, the more you use your router, the more you'll appreciate having more than one....)
Just like automobiles, everyone has a favorite, and everyone has a horror story that goes with every brand. You pretty much get what you pay for, so keep that in mind. Find a router that fits your hands, has controls where you can easily reach them, and so forth. Don't be suckered in by extra bells and whistles you don't really need, or buy something just because it matches the color of all the other tools in your shop.
Get a good book like Patrick Speilman's New Router Handbook, or American Woodworkers Woodworking with the router (I think that's the title) from your local library to make sure you are up to date on key safety concerns, as well as get an idea or two for some shop made jigs to get the most out of your purchase.
Good luck, and let us know what you end up getting!