I think somebody else posted this question a few days ago so look for my reply there too just in case I forget to mention anything here... But basically if it's anything like Stadium was, the default ones they give you aren't the best. I dunno how battling online will be OR if you can even do that part without the DS. You probably can but my guess is you're right and it would be pretty hard cuz everybody would be way ahead of you.
My advice is spring the cash for a DS. I got mine about a month ago and I've played every single day. I've put about 50 hours into Pokemon. I really got Pokemon with the intention of being able to play Battle Revolution with some good Pokemon but the game itself is so good that it's not at all a chore to be training them up to higher levels.
And I should also mention I was in the same boat as you... Before Diamond, I hadn't played any of them since yellow (which was in the same generation as red/blue) so I thought I'd be left out, but it really doesn't make a huge difference like you'd think. You can probably get right back into it.