Hi Dippy,
Not a hijack at all - it's totally relevant.
The avant bit was not a major issue, but simply that my preferred body style is saloon or coupe, and as you say you already have an avant so not an issue for you at all.
The "long and hard thinking" basically covered the following areas :-
1. the status, rarity, road presence, etc of the RS4;
2. the costs of the "move-up";
3. how about an RS6?
I have to accept that by going my chosen route, the modified S4 (even with RS4 front bumper and other "cosmetics") will never be as planted on the road as the RS4 and nowhere near as rare.
The cost exercise was fairly simple. Your car must have similar mileage to mine as £22k to £25k was the sort of range for a stock or stage 1 RS4 for a similar mileage car to my S4. As for selling or trading in mine, I felt £10k was about the top end of the range given the decline in S4 prices in the last year or so.
So I thought "hmmm, I could do an awful lot with the S4 with £12k to £15k"... Looking at higher mileage 2000 models, normally stock, you can pick them up in the £16k to £18k range, but even then that's still a £6k to £8k trade-up cost. Then, as you say, it would be rude not to go to at least stage 1, then there's the stock brakes and suspension....etc,etc...and before you know it, the real cost of the trade-up is well over £10k.
As you will know, the MRC400/450 route will cost in the region of £3,500 to £6,000, depending on how far you want to go. In my case, I have already done some of this (AWE ICs, exhaust, RS4 y-pipe) so this was another factor in the cost consideration for me (and those mods already on the car increasing its resale value by about £3.50!)
The only other thing that got in the way was an RS6, as these can now be picked up for sub-£25k, and there's not much I would want/need to do with that other than fix the DRC, plus the appeal (for me at least) of an RS saloon, but that's back on the back burner for now...
So I guess the calcs would affect you in pretty much the same way as me, assuming you put a similar value on your S4.
As regards a stage 1 RS4 being "better" than an MRC400 - yes, in terms of looks, road presence, rarity, etc, but as far as power is concerned, a stage 1 RS4 would typically pull about 420bhp/580Nm, but I'm expecting mine to do a little bit better than that.
Hope this helps.