Mezuzahs should be checked twice every 7 years on a private property ie: a house and twice every 50 years on public property ie: beis midrash and shul. Rambam Hilchos Mezuzah Ch. 5.
If you purchases all your mezuzahs at once then it would mean checking them all at once. The time span is from purchase not from when it was written.
Al pi Kabbalah, the room that you attach a mezuzah to has a connection to the room and each mezuzah should be marked which room it came from. There are many stories of people C"V ill and finding mezezuah's pasul in rooms they frequented. I was told by a sofer that the highest quality mezuzahs should be on the Front Door, the Bedroom, the kitchen, and so on.
The case and doorpost are not worth as much as a mezuzah that ranges from $50-$300 a piece. A mezuzah case usually plastic or metal can be replaced for much cheaper and doorposts can be repainted.
Make sure to protect you mezuzahs that are in direct sunlight or subject to rain with sunproof and waterproof casing. If you have a hallway door that leads to other rooms including the bathroom and that mezuzah is visible from inside the bathroom, there are opinions that say that the mezuzah should be covered with a solid case and not a clear one.
You do not recite another blessing when reaffixing a mezuzah that you checked if you put it back up on the SAME doorpost the same day. 2 days-30 days is a maklokes. Everyone agrees that you say a blessing after 30 days. Here is a Tshuvah on the topic. Sefer Otzar Sefer Torah, Tefillin and Mezuzahs, by Rabbi Yisrael Asher Krauz
Most Sofrim are able to accept FEDEX. My parents sent mezuzahs to New York to be checked because of a well known sofer there.
The Lubavitcher Rebbe told his Chassidim on many occasions to check Tefillin and Mezuzos at least a year in Elul. He spoke at length about making sure every Jew has a kosher mezuazah in the mid 70's.
Sources on Mezuazah. Y.D. 286, Rambam Hilchos Mezuzah