Unfortunately, after a few months of colonization, lava rocks are no more useful than large boulders for surface area. While their surface area is initially greater than that of a smooth stone, it does not take long for enough mulm to fill in the pores leaving a practically flat surface. I would recommend medium-to-small pea gravel as a viable and comparable alternative. You'll still be in the same price range but with more surface area exposed in six month's time.
For the record, I have used lava rock with good success in lightly stocked tanks with really good mechanical barriers before the water reached it. The pores will stay open longer if most physical debris is prevented from reaching the lava rock. I use nylon paint strainer bags from the paint store as a sort of filter sock/filter membrane in most of my tanks and it holds up for mechanical duty for at least a year (with weekly rinsing) before getting too clogged/fuzzed over.
No, lava rock shouldn't affect pH. Most of us around here nave figured out the need to buffer, though, as our tap water crashes from 7.3-ish to about 6.4-ish overnight. Whether you use crushed coral (the easiest go-to solution) or sodium bicarbonate (good 'ol baking soda, add to new water) seems to make no difference to the fish. The goal is avoiding the pH spike that would happen as the tap water degases.