Slowness could be due to dust causing overheating resulting in cpu thermal throttling. Once the laptop internals are cleaned, it would help. You could also do a defragmentation of current drive but that wouldn't really make a much difference.
Regarding battery, you can check in the dell service center. It may be pricey but you would get the best there.
For SSD, are you planning to keep your old harddrive. If so, you need to get a caddy case as well. Would cost around 300-400 INR. On amazon, silicon power ssd is currently priced around 2.3k and reviews are excellent. I would go for it. I have installed ssd in my laptop as well but different ssd, I believe I have WD.
Let me know if you need any more help.