Hi Soviet,
I think I would agree with HB.
The great thing about the ST kit is that you can build it stock, then upgrade bits later on with the minimum of modifications to the guitar.
That being said. You should definitely replace the plastic nut with a bone nut....the tone difference is huge. It cost about $27 for a PBG bone nut.
The most expensive upgrade (arguably) are the pups. but its the upgrade that you'll get the most out of. Look there is nothing wrong with the chinese pups, they have a ceramic bar magnet across the single coil poles. the gauss of the bar magnet is quite strong on the ones I've seen and the copper wire winds are a bit over wound. They are not wax potted either. Look, they work (for a start) but they have more of a tinny kind of tone instead of warm tone and they can feed back (squeal a bit with more gain).
There is a great comparison video done by Gav comparing the stock ST pups with the Bensons.
http://www.buildyourownguitar.com.au...ead.php?t=5589
Kimball posted some links to videos of other pups that Pitbull sell here
http://www.buildyourownguitar.com.au...ead.php?t=5676
The electronics - specifically wiring, switch, pots and capacitors are the cheapest upgrades. again there is nothing wrong with the chinese pots and switches - sometimes they are scratchy and eventually they will wear out. You can get CTS pots for around $7 or Gotohs for about $4 each, the rest of the electronics cost is quite small. Its hard to describe the benefits of upgrading the electronics till you compare yourself.
The Chinese tuners are fine. I find that they can be a bit hit and miss with the consistency of the turns and the tightness of the gears. If you want a cheap upgrade, the locking Grovers that PBG sell are only about $45. They are mint!
Look mate, its all up to you.