You can get good tyres for less than £500 just do a search on here.
As for baby seats, well, i held out until my son was 9 months old and splashed out on a Recaro young sport forward facing seat, (9 months to 12 years). Not cheap at £150 but it is very safe. I use it in the zed now and again to take him to his Nans on a sunday if its a nice day, and he loves it. The passenger seat is right back and is covered so the baby seat doesn't mark my leather. The airbag, if deployed, god forbid wouldn't come near him as he's that far back and well restrained. I'm happy with it, and obviously i'm extra careful with him in the car and drive like a typical sunday driver.