I am going to purchase my first FA1 DIESEL and want to know what size curve it will work with

I am going to purchase my first FA1 DIESEL and want to know what size curve it will work with

I currently have mostly 4 foot diameter curves but I want to go ahead and get into larger engines. I can expand my curves by inserting straight track pieces into them.
I use rail clamps and cut my track with my band saw to retrofit. I have 400 plus feet of laid track outside. I have it secured on 8 inch wide treated wood and dont
want to rebuild my 10 curves with 8 foot radius curves I will have to restructure
my layout.Unless any of you have a solution that you have used to prevent this? I understand a 5 foot radius curve works with most larger engines. I have been learning as I build and used 8 foot diameter curves on my third track layout but I would like to run them on my older layout without totally restructuring it.My thanks to anyone who has advice to give me and would also like to know what larger diesel engines are most reliable I am thinking about an FA1 OR A NW2 DIESEL FROM USA TRAINS but if you know a better engine let me know