Roots can intrude into the slightest crack in PVC pipe as roots seek water during dry periods.
Sometimes the internal valves in toilets wear out or become intermittently inoperable due to mineral deposits. In theory they can be replaced but in real life it is most difficult to reassemble without toilet leaking. When I worked in plumbing we always recommended replacing the entire toilet rather than the valves as being the low cost solution.
New one gallon per flush toilets are as good as old five gallon toilets. I have Kohler CIMARRON toilets ($225 +/-) in my five year old custom home and both have been faultless. Almost everyone installs high 'handicap' toilets these days. The old, low, toilets are too difficult to rise from.
If you take off the toilet be sure to have TWO wax rings available. Do not torque down the toilet to floor bolts too hard, toilet bolts are engineered to snap before the porcelain will crack. If you decide to take up the toilet I would install a new one. Toilets do not last forever. If your floor is uneven you may wish to run a bead of clear silicone calk around base of toilet after installation.