The only thing you will see by pulling the bottom cover is the ring and pinion gear. However it is important to do that inspection. Check carefully for damage and/or pitting. If the gears look good I think the market value of the ZF would be around $3,500.00 to $5000 However if you see damage and/or pitting the rebuild bill adds up fast. I had to buy a used ring gear for my ZF rebuild and it cost me $1,600 just for the gear.
Looks like you have the basics down as far as inspection. If the ZF checks out ok and you buy it plan on spending $2,000.00 to 3,000.00 on the rebuild and safety wiring of the ring bolts if there are no other major problems once the ZF is disassembled.
On a side note I know a few people that have spare ZF transaxles and none have needed them. They are a really reliable transaxle if maintained. The used prices are high because of all the kit car guys buying them up.
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