Some specs of the 04/05 cars also have a light grey carpet which doesnt go well with the dark anthracite plastics and trims, where most later ones have a completely black carpet.
The easy way to tell an early car is even the base model ones have dark headlights like the GTis, they also have a 4 spoke steering wheel, and the gear lever gaiter clips directly into the console, it doesnt have a plastic trim surround to clip into which then clips into the console. I think the 55 plate seems to be when they changed from the rubber coated plastics and so on.
TBH, they are common enough and cheap enough now that a later one shouldnt be too expensive, especially because even the latest mk5s are now 9 years old. I also dont think there were ever any 53 or 04 plate GTis, possibly not even 54 plates, earliest GTis ive seen are 05s.
If you go for a diesel, seemingly the CR TDis are much better than the earlier PD TDis, however the PD ones dont have a DPF to cause problems which to me is a massive plus point for buying a PD (I own a CR 2.0 Jetta and have had a few DPF faults and issues)
Dont get a base spec, still well screwed together interiors, but even the midrange ones feel vastly better to sit in, Match, GT or GT Sport spec would be my choice. S, Sport and SE are a bit too basic for my tastes. things like lowline display, no trip computer, no check control (low oil, bulb failure warnings etc.) no rear electric windows, no twin sunvisors with mirrors and lights, no ambient lights, rear armrest, front armrest with rear heating ducts, no rain sensor, auto lights, auto dim interior mirror, stuff like that which just make it a nicer place to be.
All the better if you can find one specced from new with options or even already modified with upgraded stuff, things like highline clocks, MFSW, MDI, bluetooth, better stereo like RNS315, RNS510, RCD510, because like most of us will want to add it.