Shop around. One agent in Pattaya quotes 37,500 all in, and another quotes 12,500 for visa only, or 17,500 if you need visa and bank account (so I deduce 5k for the bank account)
don't forget, subsequent renewals you can use the income method. Instead of the 800,000 baht, you need to show "steady income" of at least 65,000 every month (confirmed at Chonburi Immigration in Jomtien yesterday - other offices may differ)
Once you've lived here 180 days, you are liable to tax assessment on the income you bring in to Thailand. But you can off-set tax already paid on that income. If it's just pension which is taxed at home, there's usually nothing more to pay.
But note, if you are resident in Thailand, they are considering making ALL your worldwide income taxable. However this is not currently the law, just maybe a serious proposal to watch for news on
By then you likely can take your trip to Nepal half way through the 4 months, and then come back into Thailand without a visa for the remainder of your stay
Buy it at a local pharmacy. Or mail order if the price is even better. (yes of course you can post meds, but you don't want them sent from UK and stuck at Thai customs)
I get my heart meds locally. eg Ranexa 1,900 baht for a box of 60. It's the most expensive meds I take. Most are 100-400 for a month supply