Expand a French business or become an employee with a UK company?

I set-up as an auto-entrepreneur in 2011 and this structure has suited me reasonably well for the last 6 years. As a virtual assistant I have many big and small clients, and they are predominantly in the UK. In the last year the work with my biggest client in the UK has increased, doubling it since 2014 when we started working together. I’m now at the stage of wanting to expand my business because I will be heading towards the AE limit this year and I would also like to outsource more of my work.

My question are:

1. What structure do you think would be suitable?
2. I don’t currently have a lot of expenses, but as I plan to outsource more, would this come under expenses?
3. I also need a new computer, so I hope that this might count as an expense too, I’ve seen €500 is the limit for an expense, I will be getting a Mac which is in the region of €1500, how would this be seen?
4. I’ve read through a lot of the questions on this forum and I think the reel simplifie might suit my situation, if I were to change to this in January 2018, would I start charging TVA on invoices even though technically there is no way of knowing if I’ll go over the AE limit that year?
5. Another question I hope you can help me with. If my client decides to give me a work contract (he’s in the UK and I’m resident here) as an employee of his limited company, would I need to pay social charges on this or just tax? It’s becoming increasingly obvious he may need more time and as I have to have multiple clients, I can’t just give him more time and get rid of other clients. If he did this, I would then keep my AE status and do some small projects around his work, but I need to make sure I am not missing something obvious here and I am going to end up losing out financially.