First time poster here looking for guidance on whether to purchase or pass on this vehicle.
On this 1997 Def. 90 300 Tdi, you'll see a cracked chassis and I'm wondering if anyone has experience enough to tell me if this is something that can be cut away and replaced or if I need a whole new chassis? If the cutaway is possible, what would that cost (approx.)?
The vehicle is going for $26,500. Is this work worth the investment. I'm not looking to ever sell this and it's going to be a work vehicle, not a show piece, so I don't care about optics.
Any help would be appreciated.

Sorry to say but that is not a crack. That is the result of corrosion working out from the inside of the chassis box section.
What is worse is that someone in the past has done a rubbish job of patching the area probably hiding worse underneath.
What is the rest of chassis like? You cant beat going around a chassis with a little hammer tapping all over. Nice ring sound good, dull thunk sound bad. Either way as you are not going to weld yourself best pass I recon.

That patch has covered the drain hole.
Check the other side.

It could be repaired & start saving for a chassis

Thats a proper mess of a bodged repair, which means, unfortunately that the chassis is likely to be bad in other areas. After all it is 28 years old.

Time for a new chassis. About U$12000 (fitted) in the UK

U$26,000 is a lot of money, the car itself would need to be immaculate with new doors, no signs of rust (at all) on the bulkhead.
No bimetalic anodisation on the alloy skin and a really sound engine and transmission.
Unless that has all been replaced (with paperwork to prove it has), i'd walk away.

That sort of money, less probably, would get you a good 25 year old Defender with a galvanised chassis and good doors, bodywork etc.
Shipping to the USA is around U$1600

As mentioned, a poor repair which was likely covered in underseal. The rust will also be worse than what you can see as the chassis will be rusting from the inside. There might be space to negotiate on price given the cost of repair but unless you can weld, it won’t be cheap and the rot will be worse than you think.