The only limitation of selecting a particular nation for a character is starting location, initial faction standings, and maximum faction standings. Beyond initial standings a player of any nation can increase and decrease faction standings via missions and botting within the respective faction's controlled space.

Itani/Serco characters start off with Itani/Serco Admired (+600), Serco/Itani Hated (-600), and relatively neutral from Dislike (-200) to Respect (+200) with the other factions, most are Neutral (0). UIT has the benefit of not immediately having a faction standing of Hated with any faction.

Each nation has a few faction specific ships offered for those with high enough standings.

Read the wiki if you haven't already.

EDIT: Removed incorrect information.

Did they revert the faction change which prevented anyone but itani from buying the valk and serco from buying the prom?

No, they haven't. Surbius is misinformed. The faction you choose limits your nation faction to being the only one of the big three that you can achieve Admire or higher (+601>).
A consequence of this is that only people of that nation can buy the special ships attached to them.

lol, maybe you should login surbius

As far as I know you can buy the lesser nation ships with +600 on a foreign nation (SVG, IBG), but not the main ships (Valkyrie, Prometheus)

As a uit player you also have the choice to join either military. The servo or the irani. Hit does not have a military.

Once you know how the game works, it's not hard (and can even be fun) to roll an alt for whatever nation, so it's not a big deal.

Six character slots, you can try all three and have room to spare.