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).
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.
The biggest tanks need more than one player, so yeah, in team play multiplayer only.
I think you can spawn it in instant action with bots as well, at least on the Onlslaught map type.