Ramping a site up to PR6 might be straigntforward - if time consuming - but doing so without a linkspam penalty is extremely difficult.
I am guessing you just plan to blast the site with a stash of high quality, relevant high PR backlinks? If you have to apply all of these that you have at your disposal in a reasonable timeframe (6 months) you will struggle to evade a penalty. I feel that to get a safe PR 6, it'll take a year, as you need to slow down the backlink building to come out safely on the other side.
Why do you need to get the site to PR6 anyway? It would only matter if you were building it up for SEO services or out-linking purposes.
You can get all the traffic you need with a PR 3 or 4, or even less. It seems like a bit of a pointless goal to me to just say "I want this site PR6" and then expect all your client's financial worries to go away once it hits PR6. It won't necessarily happen.
PR is not a cause but an effect, based pretty much on the number and quality of links you have coming in and not much else. I have a number of PR 4 and 5 sites that I hammered with backlinks, but they get zero traffic due to penalties. However, they still keep their PR, despite being essentially worthless except for linking out.
