I'm a relative beginner, but I think I can comment.
I have a 20 button C/G anglo Lachenal , which was already updated to modern tuning. Mine was (is) in excellent condition, and I love playing it, and I love the sound. The light comfortable feel, lovely woodwork and sweet tone are what sold me on my Lachenal 20 button, despite the limitations in keys.
But if you want to play Irish you will really need those C# reeds, to play in D major and you might be wanting an occasional G#(Ab) to play in A major. I know I miss them. and am constantly tempted to buy another instrument to cover that need. I probably will some day.
OK, as for C/G vs. G/D - It would make sense that the G/D would be better for playing in D, but from what I read, the traditional style of Irish playing is with the C/G played across the rows to get D, so the phrasing will be distinctly different. But playing across the rows like that assumes you haves a 30 button or at least 24+ buttons
Yes 24 button instruments are available from time to time. Most I've seen are old and in poor condition, or else well out of your stated price range, but you might find a gem.
For C/G 30 button, the Stagi can be a good instrument. There is some significant variation in quality, but if you can check that you are getting a good one, then there you go! But do make sure, because from what I read, there are some that aren't that good, and at least need some adjustments. Some people don't like the sound of the Stagi that well, and I would agree it doesn't have the sweet tone of my antique Lachenal, but on the other hand I have heard Bertram Levi claim that the Stagi sound blends better with the sound of a fiddle, and I have a recording of his that makes me tend to agree.
The Rochelle is a good choice for starting out, and often recommended. These are said to be generally good quality, and several shops that sell it will offer you a full purchase price credit if you later move up to a fancier instrument from the same shop. Sounds like a good deal. Personally I found it a bit heavy and clunky when I tried it in the shop, even as a complete novice. and the Stagi suffers a somwhat from the same faults. These are what still keep me from buying one now, as I really do appreciate my lovely little 20 button, and wouldn't want to move to a clunkier feel.
As for buying an instrument that needs tuning, but is otherwise in good repair - do be sure to check your budget! if you are really limited to £200-£400 for the instrument, then a sginficant fraction of your budget could be used up just getting all the reeds in an old instrument updated to modern tuning, or if it is instead a matter of just a few reeds being badly out of tune, I guess I would question what condition the instrument is in that caused those reeds to get like that. But I don't really know what it would cost, so maybe it that is OK. Just worth checking out what tuning really costs before you buy such an instrument.
Edited by Tradewinds Ted, 12 August 2013 - 11:26 AM.