If you can swing the extra $250, buy a P2 instead with Ortofon 2M red -- especially if you think you'll want to upgrade the counterweight, subplatter and platter later. Much more flexibility this way and the stock P2 will outgun a tweaked P1 right out of the box.
The glass platter will make a difference in terms of definition, detail and pitch stability on the P1 or P2. You might also try an acrylic platter, just be sure it weighs the same as the OEM Rega version. BritAudio sells an acrylic platter for only $59 but it's not quite as heavy as the more expensive ones from other manufacturers. That said, if you're on a budget, it's cheaper than a Rega glass platter and way cheaper than expensive acrylics.
I agree with Kirkus on the "20" stylus if that's our only option. However, the Ortofon OM series has effectively been replaced by the new and much improved 2M series, which was designed specifically with Rega tonearms in mind (as you can see from the picture linked below). Instead of paying $100 for a "20" stylus, I would strongly suggest you spend the same money on the 2M Red cartridge. It is a much better compliance match than the 5E with 20 stylus.
Unfortunately, the P1 has a slightly different design than the other Rega arms, so you won't be able to take advantage of the easy installation offered by the 2M. (You can rough it in by eyeballing a mark that sits where the third screw would be in a Rega three-point mount cartridge.) However, since the P1 shares a lot of the design details, it will still make for a very good interface.
Bottom line: on a budget table like the P1, you don't want to go crazy with expensive tweaks. You could easily spend more money than it would cost to trade up to a P2, which is a superior machine with much greater upgrade possibilities.
If you can swing the extra $250, buy a P2 with Ortofon 2M red -- especially if you think you'll want to upgrade the counterweight, subplatter and platter later.