P3Steel v4 w/ 20x30 cm bed, or 2.5.1 w/ 20x20
Go for the P3Steel v4 (20x30). The extra print area is worth it. There is a Polish supplier, Printo3D, on eBay that has the cheapest frame, and parts - cheaper than the Spanish supplier. That is where I got mine from. See Frame Prusa I3 P3Steel v 4.0, 300mm x 200 mm, which costs around £80.
This kit uses 10 mm smooth rods for the Y axis:

Smooth stainless steel rods:

2x Ø8x385 mm for X-Axis
2x Ø8x320 mm for Z-Axis
2x Ø10x520 mm for Y-Axis

Threaded stainless steel rods:

2x M5x300 mm


With respect to the Y-axis carriage, the steel carriage does add a lot of weight/inertia, you are correct. This may or may not be an issue, depending on your steppers motors that you choose1, and their torque. That said, the 3 mm steel print bed/Y-axis carriage, is ridiculously heavy, and it would be most wise to substitute it for an aluminium, plywood, or some other lightweight solution.
Apart from that the 3 mm steel frame is fine and as solid as a rock. There are a number of aluminium 20x30 print beds/Y-axis carriages available on eBay and Amazon. A thorough search should reveal a few. There are also composite Y-axis carriages, I found a supplier in the Ukraine, tehnologika_net, who, last year, had a number of different types at a reasonable cost - in fact they were the cheapest that I found.
As an aside, I built mine sourcing all of the parts separately. It was a bit of a task, but an educative one. The process certainly made me understand the ins and outs a lot better than purchasing a ready built, or complete kit, 3D printer.
I have written up some blogs regarding the kit that I purchased, see P3Steel from Poland – A tale of despair, dismay and woe. Ignore the depressing title, it really isn't that bad.
See also Heatbeds. At the bottom there are some links to various alternative Y-axis carriages. However, some of the links/items may no longer be available.
You may also find this question of mine useful, Z axis top brackets, of P3Steel, differ between v1.x/2.x and v4.

There are a number of modifications to the standard P3Steel, that may well be worth considering. In particular, you should note the Toolsen Edition MK2, see P3steel toolson edition MK2 (in German), and P3steel - toolson edition. I have written about these, and more, see P3Steel version 4 modifications. In summary, these are:

Bowden extruders
Endstops
Endstops by Toolsen
Optical Endstops by Toolsen
Idlers by Toolsen
Extruder by NWRepRap
Lead screws
Aluminium/Composite Y-axis carriage


1 I got the Rattm 17HS8401 steppers. See RepRapWiki - Nema17. The recommended steppers are high torque:

Kysan 1124090/42BYGH4803;
Rattm 17HS8401, and;
Wantai 42BYGHW609


However, motors close to NEMA 17 size, with approximately the
following specifications, can also work:

1.5A to 1.8A current per phase
1-4 volts
3 to 8 mH inductance per phase
44 N·cm (62oz·in, 4.5kg·cm) or more holding torque
1.8 or 0.9 degrees per step (200/400 steps/rev respectively)