Bill is right.. Buy once but good, it will save you money on the long run.
my opinion regarding the mic:
Of course every singer/source will have one that fits beautifully, but there are some mics that could be great on many sources.. that said:
stay away from the new akg 414 or 214.. If you want a 414 get an old one.
Check out Violet Design: on the budget check the Black Knight out, if you could go up to 1.5k I'd seriously consider the Globe Standard (fwiw I wrote a review in the GS review section about the Globe Vintage where I also mention the Standard, below my avatar there's the link).
Violet Design was formed by the ex Blue designer Juris Zarins.
Speaking of Violet, check also out the JZ mics (violet spin off, JZ= Juris Zarins) the JZ-11 might fit a lower budget.
I had a great experience using the Milab DC-96B on a local rapper.. it's a tiny mic, but was really great! (the preamp was an Avedis MA5). Over here it's not an expensive mic, I got it for less than 1k (around 6-700 with the great rubber shockmount).
If you need something very clear and open sounding you might want to look into a Brauner Classic, maybe used for saving some pennies.. very nice high quality mic.
Also Microtech Gefell could be a good option: the 930 (among other more expensive models) could be a good buy..sometimes they pop up used.
An old Beyerdynamic 740 could also be a good buy..
Regarding preamps, the ones in the duet are clean and decent, I don't know if the ones in the rme ucx are better.. they should be around the same..the only reason I'd not go with the duet is that you could only use it with macs, rme works perfectly with both mac an windows.
Getting an outboard preamp will also make a difference (as everything in a chain), but again you might want to get some great quality pieces, your mic is going to apperciate that
BAE DMP is 1k new and it's the same preamp found in their more expensive 10xx series, with the DMP you get also a killer sounding DI.
Or try to get a new or used lunchbox of some sorts (maybe the adesigns hr500 2 slots) and get an Avedis MA5 (probably the best preamp made for this format), you'll thank me later
(always fwiw you could check my review in the GS classifieds).
You could go cleaner with the John Hardy M1 with the output xformer (you might find 1 channel for about 1k), or again in the 500 series a Forssell smp-500 very clean but not harsh.
All these are very high quality pieces that could be found for a reasonable cost all considered (in some cases is a sort of bargain IMO).. you can't get much better than these at any price IMO, of course there are some tube based designs that are nice and more expensive and of course it's also a matter of taste/goals, but seriously these pre's are among the best you could buy.
All this is probaly above your budget, but try to make "futureproof" purchases, like with your inteface, get something that will last..
Then you could buy slowly, the mic first, a preamp later and so on.. Try to test yourself all this stuff.. listen to the mics, the preamps, etc, if you can..find a dealer or a studio or a fellow gslut near you where you could audition these products..
I hope this helps somehow,