In a nutshell,
Look at the new 27 inch screen iMac models.
Get the fastest optional CPU,
Decide which optional GPU you'd be comfortable with.
Get the base 8 GBs of RAM and then purchase extra, cheaper RAM through third party Mac RAM sellers.
Then add as much RAM as you think you need. The 27 inch screen models can take up to 32 GBs of RAM.
Decide on optional storage sizes.
If you need a CD / DVD optical drive, these are no longer built-in and you will need to budget for a good external optical drive.
Budget for 3-year extended AppleCare warranty service.
In case of reliability issues with the first 5 years of operational life, the iMac will be covered with free warranty service for the first three years.