Bev,
1) It's a TGV (= Train à Grande Vitesse = High Speed Train). They are run by SNCF (= Société nationale des chemins de fer français = French National Railway Company).
2) Do not confuse customs and immigration
Immigration = Check on people, whether you are allowed to enter the country and if so for how long. This is where they examine and if necessary stamp your passport.
Customs = Check on goods, whether you are trying to bring anything into the country which is banned, or which needs customs duties (taxes) paying.
When you land the sequence is: Disembark - Immigration - luggage pick-up - customs - exit to public areas.
Immigration is the main problem. Wait can take 10 minutes to over an hour.
Customs has to be after luggage pick-up as they are concerned with what is in your bags. But, customs takes no time at all, you walk just walk through the green *nothing to declare" door. They only stop a random selection of people. 99.9% just walk through without stopping.
The usual advice is not to buy tickets in advance for a fixed time train after an intercontinental flight. Too many things can go wrong, The flight could be 1+ hours late (common for an intercontinental flight). Immigration could be bad (over 1 hour). and you still need to get to the station (inter-terminal transfer). Just buy at the station and accept you have to pay the higher price - sorry.
I did a quick look-up for trains for a random date and saw the following:
Aeroport Paris-Charles de Gaulle TGV 08:59, Reims 09:52 - The one you can get if everything is on time
Aeroport Paris-Charles de Gaulle TGV 11:21, Reims 12:12
Aeroport Paris-Charles de Gaulle TGV 12:44, Reims 14:31
Aeroport Paris-Charles de Gaulle TGV 13:58, Reims 15:33
And more trains after that, so you have plenty of choice of trains
Insurance Third party insurance (insurance against damaging other cars and buildings and other people) is a legal requirement and will be included in the price. Insurance for damage to the hire car is between you and the renting company. I always get that, as otherwise any small dent is expensive. Insurance against injuries to you, or your fellow passengers, or lots luggage is your responsibility.
Hope this all helps.