I would recommend a 27" iMac with 8 GB of memory (to be upgraded by you to 16GB for less than ½ of what Apple charges via MacSales.com or Crucial.com) and the largest SSD drive your budget can afford, 1TB would be fine and last you a long time.
Since the new Mac will have Catalina on it many of the apps which are 32 bit on your old Mac will not work with Catalina. Download and run the free app GO64 to find out which of those apps will have to be upgraded or replaced with an alternative.
I run 27" iMac with 16GB of RAM and have no problems with memory with large Photos libraries (57,000+ photos). I've edited large (350mb) images and have had no problems with that much memory.