Value of no start immobilized X5?

[1BADM6]

So I was perusing craigslist as I often do looking for deals and ran across a re-posted ad I saw about a month ago. A 2001 X5 with 160k miles, supposedly in good condition, we all know how that goes, the issue is it won't start. The owner believes the immobilizer is the issue and I'm thinking they are partly right. The key is no longer valid. On the ad they claim the dealer quoted $800 to $2000 to repair. Since I will only make my offer on a worst case scenario, what do you figure a reasonable off would be? I have no other info thus far aside from the vehicle having a new starter, alternator, tires and radiator. Most X5's I see are indeed in good condition but without full service records I can add a complete front to back service to the list and possibly gaskets and seals. I'm thinking of an offer between $500 and $1000 but no more. I would likely flip this. Is there anything I'm missing here? There is no way I can tell the condition of timing chain guides, transmission, transfer case, fuel system, etc, etc. What do you think a low risk offer would be with no consideration of parting it out?