You are correct that you can add/remove plugins to your hearts content and reach your end goal more easily (and thus at a lower cost) I never denied that.
My questioning was that, when you are deviating a significant way away from the natural functionality of a solution like WP; when the budget is there to go bespoke; why go the WP route. The arguments are not "geek territory".
The quality of plugins for WP is erratic, they are 3rd party so you cannot guarantee ongoing support and you can introduce problems.
The issue of keeping those plugins up to date and keeping the core solution up to date is actually a serious consideration and while you can keep costs down in the immediate term it can become a major burden later.
To use one example; I came into contact with a community a fair few years ago that was running on vBulletin; this was very heavily customised; it had a completely original skin on it and loads of plugins.
To put some context to it. vBulletin was at that point a pure forum. It was one of the most feature rich forums but it wasn't at all comparable feature wise to how Invision is today.
The extent of their customisation wasn't a million miles away from the scope of Invision today. We're talking user photo galleries, blogs, a public site with gallery postings, a social networking element, news articles and a backend publishing suite, loads of custom user features and a heap of other stuff.
Basically it was taken from a pure forum to a fully blown community and article website that was deeply integrated; built from a custom made theme that they commissioned and features via various plugins from vBulletin.org; some in their stock form and some customized further.
As time went on they couldn't upgrade vBulletin (actually leaving them wide open to some serious security flaws, and also preventing the site from staying modern and current) as most of the plugins were unsupported on the newer versions; in fact they had became unsupported for some time as older plugins had become unpopular in favor of newer ones. Or because the developers of the original plugin had lost steam.
The custom templates were not supported (although that would have been easier to change).
The plugin problem however was more serious, while alternatives that were compatible with newer versions of vBulletin had become available the problem came down to data compatibility issues.
The benefits when they started out suddenly became a huge burden as bringing it all up to date became a big cost issue and in some cases were data compatibility show stoppers.
What's important to consider with 3rd party solutions is that you do need to keep them up to date, as the code is known and thus exploits can be found more easily. There are a heap of other things too; but that could be considered "battle of the geeks" territory.
With a bespoke solution you don't have the open code issue to worry about and if the application is well developed with consistency then it's easier to keep modern and up to date through ongoing development than a pre-built solution with loads of extensions that underneath all the gloss create a rainbow of coding styles, code and data structures and standards.
4. Yes this is how it works customer prepares a brief, they send that to the designer, the designer than starts to put together the concept, you go back and forth with design reviews until the client is happy, then you get the design coded. I have been to the rodeo many times and am fully aware of the process. With choosing a pre-developed template the customer can flick through an arrange of designs and see them in action on demos, get a feel for what they like and they don't like and 9 times out of 10 find something they love that just needs tweaking rather than investing time and money in the whole creative process. It doesn't mean it will be a worse solution just because it is built for general use or any worse looking it depends on the template/designer you choose.
It is horses for courses, yes original design is great and my intention was not to devalue the necessity in this industry for graphic designers. I understand their value on the right project and that of custom coded solutions. However the OP doesn't need to invest in an award winning designer to produce a bespoke design for this project, maybe for branding and on page visuals but not necessarily for the main framework. It is selling cars it needs to be clean, modern, engaging, user friendly and have the right calls to action which leans to an array of design solutions.
I'm sorry I can't actually agree with that. The primary benefits of original designs over templates are (in no particular order):
1. Identity
2. Strategy
3. Relevancy
4. Originality
Not; oh that's pretty. Templates also tend to be very, very generic. Most designers can spot a template a mile off.
May I ask why the OP doesn't need to invest into an original design (citing award winning designers is taking it to the extreme)?
5. It depends on the specific needs of the client and how much work is involved, what extras they need, how organised they are etc etc. You may raise your eyebrows but until you have a signed off brief in front of you you don't know the clients specific needs and are therefore not in a position to judge. However your reaction lends itself to proving my point, the whole thing is cheaper than a bespoke development overall which allows funds to be directed to improvements rather than the core build!
Oh I can agree with some of that. Using WP could bring the costs down; my statement was whether you were charging bespoke price for a WP build and stock template tweak; that would raise my eyebrows and rightly so.
If the WP build happened to cost the same as bespoke development due to development effort then that would be a totally different thing. But would also raise other questions would it not?
I hope this clarifies the points made. I will duck out now and leave the rest of you to fight it out

. Have fun!!
It was never my intention to fight it out and get into a battle of the geeks with you or anyone else. I was simply questioning a reasoning as I am not entirely convinced it's the right reasoning.
It might be the easiest or the cheapest; but not necessarily the right reasoning.