Hi Jamie, have you thought about using a Consultant to help you with the project? Our team at Salytics is a Gold Digital Services Partner of Marketo and we have 7 Marketo Certified Solutions Architects as well as Marketo Certified Experts on staff that take on projects just like this all the time. Obviously this will sound self serving, but after a few failed attempts getting some help and making sure the next time it is done right to me makes the most sense.
To start with some of your questions though, Salesforce is typically what people use for reporting. It is for more robust and has a lot more data through the funnel. Marketo is working hard and has announced enhancements to their reporting coming this year.
Tableau in my opinion (and many others) is a great tool for making easy to read nice reports for all your needs. Depending on what you need, it sometimes could be more than you would require but if you are looking to get more robust, this would be the ticket.
As for what metrics to report on, there is so many you could look at. My favorite is pretty basic but important: funnel velocity. We find when we build out dashboards that 3 is a good number. 1 for Sales, 1 for Marketing, 1 for Executives with about 20 metrics/dashboard. Obviously from there drilling down makes a lot of sense when fine tuning a lead scoring model but day to day too much more and you will just be overloaded and won't be able to track trends.
Chris makes good points.
My 2 cents is - Don't be ashamed to start in Excel!
Excel doesn't get the credit it deserves! It is so powerful if you know what you are doing.
A few months of excel reporting, then you can automate it onto other tools and you'll feel confident that you are looking at the metrics that matter. Rather than spending time and money configuring a tool and then you realize it isn't what you were looking for.