Once you buy Minecraft, you don't have to purchase anything else to get the upgrades and newer versions. So it isn't "worth it" to wait on that front.
And what about it's multiplayer?
Tekkit has a lot of multiplayer servers out there; the main site has a lot of public servers posted up in the forums:http://forums.technicpack.net/#tekkit-servers.25
So you can check out the different servers that people have posted there. Many are run by friendly staff and moderators that make sure you have fun, others are free-for-all worlds of 12 year olds blowing each other up as fast as they can. I have had a lot of fun playing the Tekkit modpack, and there is a lot of support out there for it. If you think any of the stuff sounds interesting, I would highly recommend it.