Once you let a garage loose on the restoration the cost will spiral. Even just a chassis change ends up much more than you first think. It won't just be the chassis and brakes, it will be bushes, wiring, suspension parts and so on. Even if you try to control the cost, when the old chassis is off and the garage tell you this that and the other need doing before the new chassis goes in, are you going to say no, put the rusty bits back on?
It looks great as is but the state of the chassis and bulkhead are all important. Often a chassis can look rusty to the untrained eye but be perfectly servicable. Look for holes especially around the leaf spring hangers, dumb irons and rear crossmember.
Col