The answer to what fat type/percentage is a personal preference, or in some cases a "this is how I learned".

But of course you need to start somewhere to form your own opinion. Start with small batches - 5 lbs TOTAL (whatever you are mixing together) is pretty good: it's big enough to have a repeatable recipe, but small enough to that if it isn't that great, it won't take you a whole year to "suffer" through it.

Start 50/50 with whatever you can get at what you feel is a reasonable price. I usually do 70% venison to 30% pork for fresh sausages. You may want higher fat content, especially if you do a lot of grilling.

For summer sausage and snack sticks - I do 100% venison. The snacks sticks will be fairly dry and don't benefit from added fat. Summer sausage has some filler/binder in the mix, making it creamier before smoking - venison alone gives a nice enough texture and flavor for me.

Just remember you can't do it wrong, and you will figure out what you like. And that the less fat it has, the less forgiving it is to overcooking.