I don’t think increasing the budget to 25k would be worth it as it will be used for calls, whatsapp and such. Even sub 15k smartphones these days have good enough performance for such tasks. Definitely get a 256GB variant of whichever phone you get if you want good storage space. 128GB storage these days don’t cut it and some manufacturers are launching phones without SDcard slot which means you can’t expand storage at all.
As for specifically which smartphone to get in the 15-20k price range there are many threads here in buying advice sub-section that are all valid that you should read.
Technically, Oppo K13 or Realme P4 256GB should suffice under 20k (latter is lighter). For closer to 25k, keep an eye on OP Nord CE5 in the upcoming sale. By spending more money, you get faster performance, 4 OS updates (vs 2) & slightly better cameras with an ultrawide. If Nord CE5 gets deep discounts, might be worth it.
Xiaomi & vivo’s UI are inferior in general & Xiaomi has worse battery optimisation vs others as well (vivo doesn’t have this issue at least). So unless you want best performance for gaming, Xiaomi phones are to be avoided in general.
Even with 5200mah battery, CE5 has better battery life. Indian Nord CE5 has 7000mah battery, so battery life will be much better.
Keep an eye on Realme P3 ultra, might get a good discount to make it worth it. Slightly smaller battery vs CE5 but has stereo speakers, is a bit lighter & IP69.
I am also looking one for my mom in the same price range, but I do not want any chinese brand phones with their good awful bloated UIs and full time spying software. I want a stable phone with a good UI like Nothing but it is inferior in all technical specs for it’s price range. Really in a bind. Ugh
How is Opo K13, heard its going to be around 15k and has good battery backup and specs. Only the cameras are not great and bloatware needs to be removed one time post startup on first day.