Hi guys. I'm moving (hopefully up!) from my trusty UA-25EX which has served me well over the past few years.
 
Considering investing in a Focusrite ISA2 preamp and an RME UCX (or UFX) to cover all possibilities - vocals, acoustic guitar, mandolin, fiddle, etc.
 
Am I going in the right direction? I have a Neumann 103, a Rode NT1 and a few SM58s.

Sounds like a nice upgrade.  The RME should have nice converters.  And it comes w/ preamps.
 
The ISA stuff is good, middle of the road single-transformer sound.  And having stereo is really, really useful. Still, since the RME (like just about every built-in preamp) uses transformerless preamps, having something thicker is also useful.  The Warm WA12 is a good candidate for this, and you could just about buy two for the list price of the ISA Two.  The Warm Tonebeast is just about the most flexible preamp out there, with all kinds of combos of transfomers (or none) and Op amps.  Finally, the RND Portico line.  I know they have a reasonable dual preamp/DI unit.  The Portico 2 is just about the biggest sound I've ever found in a preamp.  I don't know if the single rack units sound as big (the 2 runs on big rails), but I would definitely try it before I invested in a ISA Two.  But really, any of the above should work fine and give you something other than the clinical transfomerless sound of the interface's preamps.
 
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Thanks AT, I'll have a look at the Warm and the Portico.  :)