I too had a SixSixOne Pressure Suit (I say "had" because it got cut off in the Trauma Center a month ago) and I plan on getting another one before I ride again. Yeah, it can be hot, but I only found it to be a problem when I stopped moving. As long as I was riding, it was fine. You will need a bigger jersey to fit it, however. I wore a Large Thor Core jersey normally and went to a XXL with the Pressure Suit and it worked fine for me. My XL AXO jersey also fit OK with the suit. I prefer it to separate pads because the elbow pads don't fall down your arms when you're riding. That was the main reason I bought one, to be honest. After I got it, I was sold on the extra protection.
I'd also suggest knee pads at minimum, preferably something that extends into the top of your boots. A friend had one ride up in a crash and he took a footpeg to the shin in the process, which wasn't fun at all. When I broke my femur the knee pads saved my knee from damage, on top of the broken bone.
You said you have a helmet already; if you don't have goggles, get some. Your eyes are worth the $25 price tag. Enough said.
Now, if you can afford it, I'd also suggest a set of Asterisk Cell knee braces. Yeah, they're $500 or so, which I realize is a lot of cash, and your knees may be fine right now. Ask anyone who's had a knee injury if they'd pay $500 to undo the damage and I'll bet dollars to donuts that they'd jump at the chance. Better to try and prevent the injury in the first place. I've got those on the "to buy list," after I get a new Pressure Suit.
Decent MX pants are also nice because they're cut to fit knee guards/braces and most have leather patches inside the legs to help you grip the bike. You can typically buy last years models on closeout, with a jersey, for reasonable prices. I've got two sets myself (the third set fell victim to the doctor's scissors in the Trauma Unit, the first time I wore them no less) and it is nice not having to put on funky, dirty gear to go riding two days in a row or before your other set dries out.
The most important factor to consider is that this stuff is protecting your hide in a crash, meaning it isn't a really smart place to try and pinch a few pennies. Those carbon fiber fork protectors don't do you much good in a hospital bed, if you follow me.