I've done this myself and if you can get it up on axle stands high enough to change the exhaust, it's high enough to underseal the arches and chassis.
Thankfully mine had bare minimum rust and 5yrs on the protection is still great with lovely water behaviour. I would definitely recommend buying £80 worth of product and having a go yourself setting aside a couple days. I've used waxoyl and never given me a problem with good prep as any product is only as good as the prep. Granted better products have come out in the last decade, but that's not to say waxoyl hasn't changed their formula since release.