Hi Pani,
I see I have a few typos from autocorrect, sorry about that it was late at night when replied and missed them.
I've bought and sold many Garrards in the past. Sonically there isn't much difference between the oil and grease bearings but the grease versions seem to be more collectible and people pay a little more for them, specially the grey hammer tone ones.
Im not a fan of mods on these tables, even simple things like mats I prefer the original ones. Its not that they don't work, they mess with the overall balance of the table and that's a big negative for me. The only upgrade that I heard made sense was a heavier platter but it all depends on who made it. I'd start with a completely stock unit to begin with and go from there when you know what they're supposed to sound like.
I don't know anything about the Gray Research arm but the other 3 are about the same level product with differing qualities, you won't go wrong with any of them.
david