Ask HN: SaaS Stack advice for non web-dev

Please excuse that this is a fairly common question, I have been through all the threads I could find on this topic but none touched upon these specifics. I have many years experience working with low-level languages, so this is a bit of a maze!

If anyone with significant experience in this area is interested, I would be happy to pay for an hour of Skype consultation.

I am at the early stages of building a SaaS that has a few properties.

1. Very large SQL database on the backend 2. Database updated from numerous sources daily 3. Expensive computations performed on the data, server-side 4. Relatively complex visualisation of the results

My preference would be to use as few languages/frameworks as possible, but I am fearful of introducing serious technical debt due to choices like these made early on. So my questions.

1. What stack that has the fewest moving parts might be suitable?

2. Is it possible to have a 'shallow' stack that covers back to front-end (except the JS bit)

3. Would Python with Django be suitable for my requirements above? My concern lays with the ORM, which may have performance issues with the amount of data.

4. Does using the ORM result in significant duplication of efforts? SQL tables are complex and have already been defined.

5. If I was to use Python/Django, could I bypass the ORM in order to utilise the useful Django features?

Many thanks for any help with this. I have done a huge amount of work on the database, but have spent the last 5 days absolutely bewildered by the choices available.