G'day Choke and welcome aboard mate.
First up, It is possible to paddle from Kenilworth or thereabouts to allmost Maryborough when the river is flowing as it is now. there will be some portages involved. (carrying your boats and gear around wiers and very shallow patches etc.)
I am by no means an authority on the upper Mary but have paddled parts of this magnificent river at times.
There are no kayak or canoe hire mobs that I am aware of in this area who would cater to your needs. You and your mates would probaly need to supply your own boats. Perhaps hire them from somewhere else. Canadian style caoes would, in my opinion be a better option with their superior load capacity.
As far as I am aware, there are no shops within walking distance from the river except at Gympie through which the river flows. You will need to be self sufficient in everything. the water is fresh and clean and I drink it. Others have experienced "difficulties" in that they have got the shits after drinking it and blamed the water for it. The choice is yours mate. I would boil it first at least. Plan on travelling a maximum of 25 kilometres per day. Some days you will be very lucky to get that far.
Mate, paddling a surf ski is very different to a multi day/week expedition in canadian canoes or kayaks - particularly so, through wilderness and genuine grade 3 rapids in places. This is a huge undertaking by any standard and is not to be taken lightly. You will need to carry enough food (and the means to cook it) for at least a week. Be aware that the river rises rapidly and is very capable of washing your camp and boats away while you sleep.
There are many publicly accessable places to put in along the length of the river byt vehicle security will be a problem unless you have a road crew. I would very respectully suggest a day trip along a section of this river to at least give you an idea of what you are planning onundertaking. To this end, i would recommend the section between Gundyah and Tiaro.
This is a fairly big day's paddle and you mates will get an idea of what they are about to attempt.
There are plenty of places to camp. be aware they are all privately owned by farmers who don't seem to mind much provided that the campers do the right thing and leave no mess or fires burning.
If you go here :-http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDQ60287.html
and scroll down to the Mary River at Miva. The river height needs to be higher than 1.8 metres and less than 2.3 metres for optimal, safe paddling in that part. Any lower and you will be doing a lot of walking, Any higher - suicidal IMHO
Wildish bass are in this river in numbers as are toga and catfish and mullet. You will see lungfish and the Mary River Turtle and the trip is well worth doing but you need to prepare for it . Good luck mate.
Here are a couple of piccies of the river between Gundyah and Tiaro.