HASL,ENIG,ENEPIG,Immersion Silver,Immersion Tin,Flash Gold,Golden Finger,OSP,Lead free HASL etc. Surface treatment only affects the reliability and cost, normally ENIG is ok.
beside reliability, surface finish requirements depends on how frequently you need to plug in /plug out board & components... Certain areas require thick plating. For edge finger (Hard gold) 50uin Au over 200uin Ni is recommended. If you use any smt socket (for your chip/ic) or any smt plug in component, it is recommended to have same surface finish. ENIG is only ~5uin of Au with 150uin of Ni.
We are talking surface finish, hard gold for connectors is a different subject and has no bearing on ENIG, if you solder a socket to the board the solder pad would have an ENIG finish, why confuse the situation!
HASL(lead free) the lowest cost pcb's surface treatment.
ENIG excellent solderability , coplanar-flat surface,excellent shelf-life,withstands multiple reflows. Higer price.
In your case ENIG (Au 2u" Ni 150u" ) is OK.