If everything is in good shape, you should be fine with the upgraded pump using the stock reservoir, with the big cooler you have. What size is your ram? However, it is nice to have the larger reservoir to increase the fluid volume some and help with the cooling. Depends on if you have the $$ for just the pump, or the whole deal.
It's good to have a filter in the system, it protects the rest of the parts if one of them goes south. Metal shavings can get in the system from a worn part, then you replace the part but the system is already contaminated so the new part might not last very long, and you end up chasing a frustrating problem. Too many of us have been down that road. I have a real nice inline filter with slip fittings that I put in the return line between the cooler and the reservoir, it came from Lee Mfg. PSC makes a reservoir that has a filter on the bottom.
Also, I highly recommend the 4.5" pump pulley from PSC. It makes a good difference on low speed responsiveness. Regular PS fluid should get the job done, but I use Redline synthetic. I had enough problems with this system before I got the bugs worked out, and I work it hard enough, that I'm doing everything I can to make sure it works properly. I have a Tommy Lee small bore steering gear, Tommy Lee seperate reservoir and filter and high performance pump, a PSC 4.5" pulley, a PSC 1 5/8" ram, and a cooler. It works perfectly with 40's, plenty of both power and speed.