Help Please Guys

I need some help in answering a few questions I have regarding retirement.
I know there are already a few topics on here its just I would like to be 100% clear before I take the plunge so to speak,
I have been quoted 25,000 Baht by a company in Pattaya (is this about right ?) to complete the process but would like to see if its feasible to do it myself or do you chaps think it is better to cough up the money ?
I am fifty-eight years of age,
So, questions below and many thank you in advance to anyone who can spare a few minutes to help,
1. Can I change normal 30 day tourist visa to a 90 day Non Immigrant visa with a view to convert this to a 1 year Non Immigrant once I arrive and can this be done in Pattaya ?
2. I have already opened a bank account with Bangkok bank. I have a monthly income of £1300 from property rental in UK and a small pension from my life companion who passed away last year. I also have savings of around £75,000. I intend to deposit the required amount to make up the total of 800K baht shortly to begin the seasoning process. What will the authorities require by way of proof of income ? I have payslips from my life companions company each month and a signed rental agreement with my tenants, are originals of thosesufficient ? Or copies of bank statement here in UK showing monies going into account each month ?
3. How long does the whole process take ? I intend to come to Pattaya/Jomtien end of July early August but I need to leave again early November for a couple of weeks to attend my daughters wedding. I understand the need to acquire re-entry permits before I leave.
4. Is it feasible to rent a property long term as soon as I arrive ? Is it likely that my application will succeed providing all criteria is met ?
5. If I did this myself approx how much would the whole process cost ?
Any other info such as what other documentation mightbe helpful isgreatly appreciated guys.
Once again, many thanks