Your best option is likely going to be the RX-7 Type RZ '00. It's tied with the Type R Bathurst R (the trim that mrxbas used to beat GTWC for his "under 25k used car" challenge run) for having the best grip and least turning resistance, but it also has wider tyres than the Bathurst R, leading to better stability and reduced tyre wear. It's the rarest FD in the used dealer though, so you'll need to keep an eye on the late-90's dealership for it.
Compared to other options like the Esprit, Mustang, Griffith 500 and Cerbera Speed 6, it has a favourable 50:50 weight distribution and very even tyre wear, and while 590hp is barely enough to beat GTWC, it means it's quite easy to handle out of corners. Will still need a lot of line-up resetting, and you'll be finishing mid-pack until you get to technical tracks like Hong Kong when you can start building up points.
These are my settings for it, but bear in mind that this was built around sports softs, so you may need to add more camber for better cornering stability. Racing brakes were also added. If the car throws its weight around too much, try lowering the height to 105 / 95.
Shopping List
oil change
rear wing
R muffler
racing brakes
brake balance controller
port polish
engine balancing
computer
custom transmission
triple clutch
R flywheel
custom LSD
driveshaft
original turbo (from tuner shop)
R intercooler
R suspension
racing hard tyres
weight reduction stage 1 + 2 + 3
rigidity refresh
Settings
tyres: hard / hard [R2 / R2]
brake balance: 8 / 5
springs: 6.0 / 7.0
ride height: 110 / 100
bounds: 2 / 2
rebounds: 5 / 5
camber: 1.4 / 1.6
toe: 0 / 0
stabilisers: 6 / 7
gears: final to 5.500, then auto to 1, then final to 2.500, move 1st and 2nd gear all the way left, 4th gear on top of the third white line, and 5th and 6th all the way right (I can't open up GT4 right now so I can't give specific gear ratios with racing tyres, but you should be able to eyeball it)
downforce: 30 / 30
LSD I: 30
LSD A: 60
LSD D: 5
ballast: 35kg at 50 balance
ASM [O + U]: 0
TCS: 0 or 1 (set to 1 if you're driving in AT)