Hmmm. Generally 7-10psi is the norm, for a windscreen washer pump, the geared variety.
Generally, if pumping into a sealed tank, you can expect to fill between 50-70% of the tank volume.
Norbert Bruggen supplies a neat range of peristatic pumps, which are self selaing and will pump to high pressure- no problem. Feedback from other members who use this system report that they can be a bit sluggish at pumping though, and they're a bit pricey too (DIY?).
I'd look for a good quality metal geared pump (breakers yards are a good source, and you want to be looking at the older cars IMHO), the plastic ones tend to live short lives in this application.