I also have a visa rentista and did not use a lawyer. As long as you show the paperwork that is listed on the migraciones website it should not be too difficult. For the first application they don't expect you to have a local bank account as you can't open one without a DNI. They did tell me however that in order to renew my visa I would have to have a bank account and it had to show the 8000 monthly deposits. However, any money you receive from abroad which is transferred into your account shows up as 'deposit', the same as a deposit you make yourself i.e. to renew your visa you don't need to show that the money was actually sent from abroad as long as you can show a monthly deposit of 8000. I have renewed my visa twice now showing bank statements of only the last 4 months before renewing it and no questions were ever asked.