What are you planning to put on the underside ? Paint or rust bullet ?
I can higly recommend Brantho Korrux 3-1, i personally think this is the best available product on the market that you can apply yourself.
This is a rust converter/primer/finishing layer with color all in one, with many RAL colors to choose from.
The biggest benefit of this, that it is not prone to cracking like other paints when it gets some age to it :smthumbup
I restored a 2cv with this stuff (which are notorious for rusting) and even after 6 years sitting outside, it's still like new,
just google it you won't find any bad review.
As for cleaning rusty suspension parts and bolts, instead of sand blasting get yourself Deox-C from bilt hamber, that's absolutly magic in stripping rust from crispy rusty parts without any effort.
this is what a part looks like that has been in a deox-c solution for 24 hours, and just rinsed off with water so no scrubbing with metal brush is needed
I'm also in the process of doing a complete underbody restoration and i researched many weeks/months before choosing products.
this is mine after 1 hour of angle grinding with a wirebrush