My own opinion is that I would NOT drive it until the problem is fixed.
Since the seller accepts responsibility for it and is willing to correct the problem, I would have it done before taking delivery.
There are several threads in the Suspension section on LT that address steering rack problems. Check a few of those out.
'05 Elise Magnetic Blue, Black, Touring, Hard Top, Star Shield, Stage II Exhaust, Arqray decat pipe, S2 Front Splitter (UK), Sector 111 Rear Tow Hook, Driving Lights, Cup Airbox, Shift-I System, LSS Dampers & Springs, Sector 111 V-Bar, Schroth Profi II 5 point harness, Exige S Probax seats, 1/4 turn undertray screws, Stealth bulbs, BWR SC with PES tune, Innovative engine mounts, SS brake lines.