Get it checked by a proper mechanic who you trust, and who has worked on one of these cars before.
Otherwise, at LEAST get a compression test done. I didn't get it done on a GT Four I bought a while back, and 6 months later the engine died, ended up costing me a LOT of money to get it sorted. The rear diff mounts are a common failure point on ST185s, so get them checked too.
Otherwise, just the usuals ... check if its blowing smoke, check for a "milky" coloured substance just inside the radiator cap (indicates possible blown headgasket), listen for any rattling, listen for knocking from CVs, check tyre wear to see if wheel alignment is ok etc etc. Also watch out for the synchro's in 2nd gear.
$5000 is ok for a good condition 185 with those km's, but its hard to find one in
good condition
