Hi Pierre,
Nice to hear you want to joint the 3d community.
Any of the Prusa i3 variant are a good choice to start, the P3Steel is maybe one of the best i3 around, take a look at the toolson edition in Thingiverse as said before.
The increase of the printing size on the i3 will cause a small decrease of the quality, but nothing to worry about IMHO. In any case, if you want to have the most accurate printer, go for the short version. The Y axis is probably theweakest point of the i3 and, increase the lenght will not help. You can also increase the smooth rods and linears bearings diametre on a later stage, designing and printing your own adaptors, which will give you a more solid structure. Btw, I got the long version, but I could not compare with the short so I can not tell you if there is a decrease of quality.
Anyway, as you will see, all the printer designs are compromissing quality on behaf of price in some way or another, however, you can get very decent prints with the i3. Don’t expect to be plug and play, I start getting decent printings after 1 month of having the printer operative (I am Electronics Engineer also), and still improving…
I thing the best advise I can get to you is to avoid any chinesse stuff regarding the mechanics (linear movement), avoid chinese LM8UU and stainless steel rods. Look for a known brand like Misumi or SKF for the linear bearings and use Hard Steel Chromed smooth rods with thight tolerance, try to source locally.
As you said most of the stores selling the P3Steel frame are from Spain, I live near Barcelona, so I buyed locally the frame, sorry I cannot help you here.
If you are really looking for big printing volume and good quality you can buy the Ultimaker 2+ extended or the BCN3D Sigma, ovbiously at a cost!!
You can also build your Ultimaker 2, look for Ultimaker 2 Tribute in Thingiverse, if you use a taller chasis you can get the volume of the extended also.
My last advise, use good quality filament like colorfabb, extrudr, our any other european or USA maker, advoid chinese stuff here also, the 3d printing is a sum of a lot things that can work well or not (including the filament quality) so as many things yu can leave out of the equation the more easy will be to locate the problems. When you have your printer working well, then you can explore other cheaper filaments if you want.
So move forward, look a lot, buy with some spares, start building and ENJOY!!!