I am not sure about your very last questions. But anything that will fit the R8's inside diameter will work in them.
Do you need collets? Yes and No. I am guessing the spindle is R8 ? You can get tooling that has a R8 shank built into it ( or what ever tapper your spindle has). Lets say it is R8, you should get you some R8 collects if so. You will always need collets. But you can also get other size collet systems which will come with a collet hold to fit your spindle tapper. For example if you wanted to use some ER32 collets you would get them with a holder that would fit your R8 spindle ( or what ever tapper your spindle has).
Drill chucks come either with the tpper all ready on them ( built in or add on) or you can get the chuck and add what ever tapper you want. You dont want to use a drill chuck to hold cutter though.
A lot of times someone using a mill such as you can get by with a few collets to fit there spindle. The buy mostly cutters to fit the few collets you buy. Ofcourse you will always run into wanting/needing more sizes so its best to get a set if you can afford. ER series collets will normally have better quality/runout specs. Meaning the cutter will run straighter which will be more accurate and help the cutter last longer.
There really is no one way to do all this. Much depends on what your willing to spend, how much time you want to give tool changes, how many cutters you want ready in a collet to do a job, and how much accuracy you want.
A good middle of the road set that will be affordable and still have some accuracy plus better usability ( quick tool changes) is the Tormach tooling system. You can get several different type cutter holders ( end mill holders, drill chucks, ER system collet holder and others) in the tormach system. Its not really what you would call cheap, but priced very good for the quality and performance you get. Probably more than you want for your needs though but thats up to you.
A picture of the tooling you have, and give us spindle taper of your mill would help give us some dirrection on what would be a good way for you to go. Also what type work you will really be doing with the machine. A homeshop/farming need is kinda left open too guesses of what your really doing.
You may find that putting some of the tooling you have up for sale will even pay for tooling you may need. Or you may have most of what you do need.
Measuring equipment, work holding and other neeeds always get forgotten until the need shows itself so there will be other things you will need that you dont expect. A mill in manual use will need even more than a CNC all though a CNC will need other things itself ( like software for instance).
So again some pics, and more info will help. But just for bottom/cheapest way a few collets to fit your mills tapper with sizes for the most used cutters you will use is the quickest/easiest way to get started ( all though slowest on tool changes). If you get very good quality collets good accuracy can be had ( like for instance Lyndex collets instead of Chinia made collets).
Really there is alot to all this, and different jobs may be more compatable with different systems. A lot depends on you and your needs.