Looking to spend around $500 and below for legs and head
Buying a 48" slider that weighs 10 pounds but supports up to 35
my load will only be an additional 10 or less
You are spending a lot of money on the slider where the design is optimised to give almost frictionless movement, and the rigidity to have you touch the camera at the end of the travel without the carriage and supports deflecting. Cheap tripods and heads are problematic for stable work with long lenses, and their frequent issues described on forums centres around flex, and wobble. Adding a slider makes this kind of thing even worse. With the pan and tilt locks on, pressing down even lightly on the end of the slider deflects it quite a bit, due to the engineering tolerances (or the lack of them). Before buying anything, I'd strongly advise trying your slider before parting with cash! A plastic slider could be just as good as a solid one on a flimsy tripod - and frankly, looking at what is available for this price here in the UK - it's not one I'd be comfy using. If you don't care what it looks like, why not consider something second hand on ebay. There are some old but amazingly robust ex-broadcast items out there. They are heavy and they are pretty ugly, but they go nowhere even if you stand on them!