There is app its calle dchecker or something like that.
You put it on USB, and go to UPDATE FIRMWARE ...
It will give you an answer wich is the lowest firmeare you can go.
If its 3.60, sorry, no can do 3.55CFW ...
All you can do is to hope that someone will do CFW for newer ....
In my country, used ones are same price as new. Or even more .
PS2 SCPH70004 softmod for UK/EU FMCB install ESR
PS2 SCPH75001 softmod for USA FMCB install ESR
PS2 SCPH70006 softmod for HK/SP FMCB install ESR
PS2 SCPH77004 chip
PS2 SCPH50004 chip+softmod+net adapter+160GB
PS3 CECH2504 4.30CFW+750GB INT+2TB EXT
PSP 2004 softmodded