Is it like it?

From what I understand, it's shortcomings were basically it's technical issues and insanely steep learning curve. Other than that, it's like the space 4X game.

Unsure about this. From the screenshots I've seen before, as well as reviews, it seems to be like the heavyweight version of Galciv, probably like what ARMA is to Modern Warfare.

Serial key.

Paypal is listed as a payment option on GG here. Or do you mean that Paypal no longer supports your country because of possible fraud? Only I've heard of that happening elsewhere.

Yes, if you like a 4X.
http://www.captainkwok.net/balancemod.php
Helps a lot with most gameplay issues.

Unfortunately the Space Empire games feels like unfinished games with a "build your own AI!!!!!" feature since apparently the developers have a real issue creating any working AI.
I have 4 and 5, I don't recommend either one unless you feel like investing a lot of time tinkering with the "game" to make it play like one.
You would be much better off with Gal Civ, Armada 2526, BOTF, hell, even Ascendancy.

They're pretty decent, but only if you plan on playing multiplayer. Like trusteft said, the AI is crap. In SE5 it doesn't seem to know how to defend against fighters, so a single self-supporting carrier will steamroll it.

Not really.
SEV is a 4x game in space. They all tend to blur together after a while.
The main difference between SEV and GC2 is that SEV has tactical battles in space and on planets that you can control. You usually won't (gets tedious), but they are there. Plus, GC2 was polished.
Beyond that, the big gameplay difference is how the galaxy map works. In GC2, it is very MOO-esque. You have a big galaxy and you fly ships from system to system. In SEV, you have a big galaxy, but the only way to enter a system is through wormholes/stargates/whatever (until you get a jumpdrive). This encourages defense and leads to long wars.
So think of it as a STALKER-esque "open world" (big areas, but specific transfer points) versus an ARMA/OFP-esque "open world" (one huge area).

GG:
Now they've added all the games, it's a Point-and-Click sale: http://www.gamersgate.com/all?q=pointclickweek

Oh seems that the option for Paypal is present again. Or, I must have missed it just now. Anyway thanks.

Strategy First is offering a special discount for Battle for Supremacy users.
Buy Space Empires V for only $2.49 before August 3, 2010.
http://www.strategyfirst.com/en/games/SpaceEmpiresV/
Enter Promo Code: uz6jitgdx
Also on gamersgate for that price

Yes. I had a similar problem last week. (I can't use MB as credit card companies here refuse to transfer funds to their service). However, the reason you cannot see Paypal as a payment option is apparently down to fraud. Basically, GamersGate have a list of products that are high priority targets for fraud. Such items will not display Paypal as a payment option (for example, most of the recent Ubisoft games and Blue Coins).
In such cases, the only way I was able to make a purchase was to have someone from GamersGate help me. But I don't think that's going to be an option all the time.
Your best bet, if you cannot use any of the other payment options is to let support know. Hopefully, it'll speed GamersGate up in providing a better alternative payment option.

D2D
Best of Indie Bundle Vol. 3 .95
D2D US. $5 coupon code : ThanksRaynor

Star Wolves 3 - 75% off at D2D
Ohh, and it's DRM free!
And it looks like anyone can buy it on the US site and use the coupon code above. It worked for me. Star Wolves 3 for 2.5 USD is a good deal!