I'd recommend Shell V-Power fuel-wise... it has additives which
help look after your engine - and my car feels much more urgent with V-Power compared to Total Excellium and BP Ultimate fuels. Never tried supermarket fuel - wouldn't want to either.
Get your car to a good garage (best to use a good VAG tuner IMO) and give it a full oil and filter service, get your spark plugs changed and any other servicable parts done too. Has the cambelt/waterpump been changed at the right intervals? Also a Forge DV and cotton air filter will
help your engine breathe a little better. If you get it remapped then an uprated TIP might be a good choice too. Don't forget to look at bushes all-round as these can go soft and won't be doing their job. Uprated dogbone mount is worthwhile too. Any spare cash left after this then get the upper and lower S3 strut braces bolted on.
Can't
help you with the seats sorry bud.