Having hard time to decide for a lead dev position in a start-up (Paris)

I am in the following situation where i got an offer to join a start-up in Paris for a lead ios dev position. The field they are working on is a mix of technology and art which really fit to my personality.

I have a previous experience of 7 years in a big company working in the US on technology such as C++, Qt with a big emphasis on UX. I got back to France and took a year off after this to create a full product on tablet on my own that i will release in a couple of weeks from now (so I have 1 year of iOS where i maxed out my knowledge there)

I am very undecided though to take this position as there are a couple of things that bother me during the hiring process:

• The technical interview was very brief, i just got two precises questions by one developer but i was really expected much more there to test my knowledge.
• Second the paycheck is low, it is about 40k Euros while i believe i could value myself btw 50k to 55k on the ios job market in France specially with my past experience in the states. I have tried hard to negociate the salary by exposing clearly the different values beside the one I could bring on top of the technical aspect (I also have a couple of experience in management of interns and got a good grasp on how to improve the software development process) but there was nothing I could do.
• They didn't mention anything about stock options that could compensate the low salary. They have just mentioned that I could earn really much more after the funding period they have received for the next 2 years and the success at that time.

They are in business since 2 years and they are not focus too much on benefits but for now on R&D.

I have tried to put into account all pros and cons on paper but i come to about the same pros and cons :-(.

Should i just drop the offer because there are so many uncertainties/risk or should i give it a try. The real benefits I could see here is the type of project they are working on and i have a lot of freedom to shape their next products. They have release two apps on the Apple Store - they have really good taste there but they seem to be lacking on the technical side. I am also worried that since they have a low salary to offer they are ok to hire B or C player. Any advice would be greatly appreciated to make up my mind!

Can someone enlightened me with what i should be considering maybe?

It depends on quite a lot of parameters. I will try to answer this in a general way.

1. If you have a family or some other responsibility where finances are quite important? This could be a risk.
2. How easy is it for you to find a new job?
3. I feel over a long run finances become more important as the work gets pretty static in nature. Do you see this as a long term project? Even if you join at 20% less, can you expect the business to reward you if you drive them to success? I always try to study my employers when I meet them. Try to see what is more important for them, business profit or quality work (but the non-technical managers don't actually understand software quality, so this is tough).
4. In next 6 months - 1 year, you might value your self at 60k euro. How will it feel waking up in the morning and going to work with 40k salary.
5. Have you worked in a start-up before? I have worked in big multinationals and now running a start-up, its just a different ballgame. There will be a lot more work pressure, responsibility, lots of questions by your employers(as now you will be the lead) and not very satisfactory salary.