Hi,
On a kickstart bike the battery isn't too critical as long as it's serviceable, and yes, positive earth.
I have a Yuasa as Thomas recommends. Does the job just fine.
On the subject of tanks, I've never seen one of those pre-painted tanks in the flesh, but the shape seems off, being too low at the back. Really kills the look of a Roadster.
Indian tanks in general have a very poor reputation with some being apparently un-useable without major re-work, but the Emgo ones (sold unpainted) are excellent.
I think one or two other forum members have bought the pre-painted ones, and hopefully they'll chime in.
Generally the price reflects the quality.
That said I bought a Fastback tank from Indian eBay seller 'pummy' and it's pretty good, but it's bare metal.
Personally, I'd be wary, and it's far better to have the tinware painted as a set.
Other info... if you still have points ignition I'd strongly recommend replacing it with electronics. Pazon do an inexpensive one which is good, as are the latest Boyer offerings.
At the spendy end the Trispark and Pazon Altair are more refined. I like the Trispark as it's all self-contained in the points housing and gives a great low idle.
Others are available but I haven't used them.
Presuming yours has a front disc brake, replacing or re-sleeving the master cylinder with a smaller bore makes a huge difference to brake performance. Essential mod IMHO.
Finally, a PCV engine breather cuts oil leaks and improves engine breathing - cheap ones from Mike's XS work fine, but the CNW one looks much more refined.
Do a forum search and there's shed-loads of good info.