What are the risks with having a tummy tuck? I have pelvic congestion syndrome

As it appears your condition is well controlled you still may be a candidate for a tummy tuck.  You will want to speak with your vascular surgeon about your desire for a tummy tuck.  If given clearance you can then proceed in scheduling a consultation with a board certified plastic surgeon.  

It is not clear if varicose veins are likely to cause DVT, but the two conditioners are certainly related. 
A number of factors (genetics, age, obesity, birth control pills, hormone replacement therapy, occupations that require a lot of standing) predispose an individual to PCS. You will need clearance from your vascular surgeon to have a TT as increased pressure in the abdomen or trauma to the area may be cause for concern so that precautions can be taken by your PS.

I have never heard of pelvic congestion syndrome. However you can always have a tummy tuck with no muscle tightening. This may avoid any issues with internal organs.