I can't recommend a shop but can offer the following based on extensive rust-related maintenance on 2 E53 3.0's in the Northeast US.

Break the plan into pieces.

1. Brake lines - Unless they have already been replaced, they are likely rusted...fine someone to replace the steel lines with Nickel Copper lines..after replacing the lines, they will have to go thru the electronic scan-tool bleeding procedure to get the brakes to work properly.

2. Replace the front subframe...I am sure you will be able to find great, rust-free, subframes in FL. Find someone parting out an E53 on facebook, or search car-part.com (search both for "Engine cradle" & "Frame - Front Section only") . The front subframe (BMW Part# 31116760277) is the same for all years E53 and all engines - I6, v8, diesel.

3. Based on observations and inspections done during the brake line replacement, make a decision on replacing the rear subframe. I have not yet replaced my rear subframe, but am thinking about it.

These 3 jobs which can be done over time, will correct most of the underbody rust issues.

I think any good mechanic that does pickup truck suspension work can replace either subframes.

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'99 E39 528i 5-speed 130k
'06 325Cic auto 115k
'05 X5 3.0 auto 173k-SOLD
'03 530i - manual swapped - 430k miles-SOLD