Not sure about the cost of a slider but, when I bought my 5er the hitch was part of the deal as well as the trailer brake. My current hitch is a Reese 2 way 15k and was around 650 installed. If you get a chance check out Forest River, they make a great product and the service is very good. Luckily I went to a better than average dealership and they have been very helpful.
Some things that might help in your decision is to have in mind what you will be using the 5er for. If only weekends or vacations find a unit with only the equipment you need. Myself I am full time so I needed a larger unit with more eqiupment. One thing that has been very handy is having a blackwater flush connection, drain the tank and hook a water hose to the conection outside the trailer and let the water run for a few minutes. No more dragging the hose into the trailer to stick it down the toilet to rinse that thing out. Another consideration is slideouts. You can go nuts with all kinds of floorplans and up to 5 slideouts. Find a floor plan you like with plenty of room where you want it. One thing to keep in mind is where the kitchen or galley is located, try to keep away from a rear kitchen find one where the kitchen is over the axles of further forward. Never had one but plenty of full timres have talked about everything getting thrown around when going down the road in the rear.
Also when you are looking at trailers check out the way the cabinets are made, I was told that you can tell alot about the way a unit is made by the way they are put together. My current one all the cabinets are screwed together the same way residential cabinets are, no staples or nails shot in with an air nailer.
If you can I suggest maybe a double slide, I find having one for the bedroom a must, it will add around 500lbs or so to the tongue weight but it is worth it.