You actually don't need UT2003, all maps and modes were rolled into UT2k4. Think of UT2k4 as UT2003 complete edition.

I played UT2004 few months ago, and I don't remember it having siege tanks and many other vehicles (although I played the campaign only, they may have been available in multiplayer).

Anyone still playing UT these days online is going to be very very good and very dedicated.
The biggest tanks need more than one player, so yeah, in team play multiplayer only.

I bought UT2k4.
what mods do you recommend?
I'm thinking about Ballistic Weapons,is it good?

Well first you go and enjoy the base game. When you get bored of it about 2 years from now, stop by and ask about mods. There are no real game-enhancing mods, most exist for people who are bored of the base game.

Honestly, playing the base game for awhile is probably fine.

Are you saying there were vehicles in UT2k3? I honestly don't remember there being any, maybe through mods though.

If I remember correctly UT2k4 was originally planned as a expansion pack adding Onslaught mode and vehicle play. Epic, in there infinite wisdom however changed it to a full reissue with all the content of UT2003 and in the process burned a lot of customers who bought the previous release. By proxy it had the added effect of splitting the fan base up between the two incompatible sku's.Hence UT99 still had the largest player base for many years after. I can't say if that's still true but it very well might be, at the very least there is probably more playing UT99 than UT3.

I think you can spawn it in instant action with bots as well, at least on the Onlslaught map type.

Now I've no idea where I've seen the vehicles. -_- I'm pretty sure I've played both, but maybe the vehicles were in UT2004.

EDIT: Although I remember a few maps, one was full of dust and sand and was capture the point - like, that I haven't seen in UT2004 yet, they must have been in UT2003 (and they had vehicles, pretty sure about that) (no mods installed).

Call me crazy, and I am not sure why at all, but I liked UT2003 more than I liked UT2004. Probably shouldn't matter, seeing as said above, it was a reissue of UT2003, but for some reason, it just felt a lot smoother, and not just because UT2004 didn't run well on large maps on the computer I had then.

In the end though, I played a lot more Unreal Championship than UT because a friend had it.

I recommend "Unreal: Gold", it's a great game, that aged exceptionally well.