I don´t really know why you "need" a PR 5. In many cases it does mean nothing. ut OK, the system is easy: You need a backlink, whether you get one free or buy it - its up on you. But "buying" can become expensive and useless at the same time. And it is clear as well, if the site is having 5 or 500 outbound links on the same page, the result will be different. So number of links is essentiell. I have a PR 5 site where I link to only 1 external and 3 other internal pages - 2 of the internal pages received PR 5, the external has PR 4. If there are 500 or 200 links, PR will certainly become 2 or 3 ranks less than the original page. And there is also one clear thing: Links come and go ... If they go, you will loose the PR.
I think it is not so easy to get PR5 ..by just having 2 or 3 pr6 or pr7 backlinks.. you have to make your seo efforts to get pr5.. stay on quality.. quality unique content.. quality backlinks.. quality matters.
Do you have a reference link or two on this Google opinion? While I agree in theory with this position I have never seen a written Google opinion on this.
Also I belive these rules useful: The more links pointing to a website, the more important it is assumed to be. Since people naturally use descriptive text when linking to other websites, Google analyzes anchor text. Search engines give a higher value to one way links to your website. The first criteria when buying links shoud be the relevancy of the web page where your link will be placed. Using your top keywords as link anchor text.
Did you not get the memo that Google is going after website that do this? What you have suggested could get a site banned if those links pass juice!
Google likes sites that are older, so try to buy an existing domain that has a page rank already, that is relevant to your product/niche & then add decent content/backlinks (of greater Page Rank than your page). But what you really need is decent on & off page optimisation to get you on the first page of results....there are many sites that come top of the results without a high page rank, provided they are targeting specific & well researched key phrases & have optimised their page for those phrases! Appearing high in the natural search results will get you traffic, not (necessarily) a page rank!