Well, you are luck matey, I did the exact same move myself - DC2 to S4 B5 for the same reasons.
S4 as you rightly guess doesn't come anywhere near close to the teg in handling stakes, its rugby player versus ballerina there.
The costs on insurance and servicing are comparable to the Integra, fuel is completely different. When you are ploughing about in the S4 "enthusiastically" that MPG drops drastically below the ITRs, it's thirsty around town too on shorter journeys IMO. Tyres seem to be wearing a lot better than the fronts did on the ITR, currently at about 12-14k on them with plenty of meat all round compared to the ITR where I would get maybe 8K out of the fronts at a push. Mods is where I am finding my cash has gone though, to the tune of about 4k I think.
Ride quality - oh gosh, worlds apart. The S4 is a limo in comparison to the ITR, lets face it MOST things are in comparison to the ITR LOL. You can comfortably cruise at 80 down the motorway without that buzzing sound which is your filling rattling in the ITR

Reliability - fine and dandy mainly. I have had a control arm to replace last MOT but its a 115k miler so to be expected. Damaged bi-pipe - which is a normal thing, nothing costly. For me its been cheaper than the ITR on parts, though I ended up with a yet another B-series gearbox failure on my ITR and my B18 VTI before it - knackered synco between 1-2 (£800 including the clutch

). The MAIN thing to think of is those K03's turbos though. You probably have in your budget there to get one with K04's already done and would be my choice in hindsight now. I have a credit card that I keep empty for when that day comes that they blow.
The fun side of the S4 comes from being able to lug around kids, dogs and luggage while at the same time shocking the hell out of most stuff that needs to be put firmly back in its place. The S4 will out accelerate the ITR by quite a ways, and will continue to do so all the way up to double and beyond the NSL. I have sampled this head to head with my brother in laws ITR (which was mine previously) but the ITR will dance around the corner way way better. All the weight of the S4 is largely up front and even with the suspension mods mine has its numb compared to the ITR, there's nothing like the feedback or that reward the teg gives you as you drop it in second on a 90 degree bend, plant your foot and feel the back twitch a little then the LSD kicks in along with the smile on your face. Damn I miss that car LOL.
The power delivery is COMPLETELY different. The ITR as you well know you have to really want to drive it and be involved in the process. The selecting of the correct gears and holding on to the revs to the limit to keep it stirred is not needed on the S4. Turbos spool relatively quickly and you just get a massive wave of torque that hurtles you forwards - pick a gear, plant foot, power. It also does it without any real song and dance, it's quiet and unexpected for passenger riders, my mate Rob will attest to that - after I peeled him back out of the seat as he crapped himself.
Should add if you are anywhere near the area I am happy to take you out for a spin, or will be at Japfest too in a few weeks if you want to look over one or talk more.