Got a new big project request but I don't have enough experience. Any suggestions?

I have 2 response to this question.

1. This huge project will take a long time to build and all of your energy. You will think of solutions to this project instead of doing your own thing. As you say, it will upgrade your resume and open up to similar large projects. However, that is a different path than the indie hacker.

2. What you should do is first tell the CEO he/she is dumb. Explain that there is a huge upfront cost to building their own internal system, the system is gonna suck compare to Mature SaaS offerings, and they will need at least one full time developer who's only job is to maintain that system, which will cost way more then paying for a few subscriptions.

It is good thing that you know your limits and came here for advice :) I would suggest that you definately take the project . It will help you to dive into the dev world much faster.
My first advice would be to write a proper User Specification before doing any coding. This may take some time (yours and clients) especially if there is an complicated business logic involved. However, it will help you to understand the size of the project and the team you would need. After that you may do some mockups and agree on the graphics. In the mean time you may start building the dev team.
The development part maybe oursourced, as mentioned by others, but I find it very frustrating and non workable solution especially if the application has certain degree of complexity.
Let me know if you need any help, as I do have the necessary experience in delivering highly comlex applications and the right team to complete it.

Be careful. I remember early on in my career I took on a project that was way above my abilities. It wasn't fun for me or the client.

Here is what will happen: you will say yes to everything, they will pay you because they believe you are the person to fix all their problems, you will fail to deliver, they will either demand their money back or drop you OR they'll be especially stupid and allow this to grind on for a while until they realise you will never deliver.

The simple fact is (as you have stated) -- this is beyond anything you have done before. You certainly cannot do it by yourself; there isn't enough time in the day.

"They don't want to use saas solutions"

This is another reason why you need to be really careful. They've already told you this isn't just gluing together a bunch of 3rd party services, it's building an in-house system that spans their entire business. They need a team of developers, not one developer with 3 years experience. The fact they have even considered it tells me they are clueless enough to assume you will stroll through it.. so if you say yes, they will probably give it a shot.

"I thought of partnering with an experienced software engineer and let them mentor me on the job in return for splitting the revenues but still not sure how to approach it."

This is still a major problem, though. You are proposing to start work on a potentially huge project with somebody you don't even know. Even if you put together an ad hoc dev team you'd still be responsible for keeping it all together.. with no experience of managing other developers.

My honest opinion is this is one of those times where the responsible thing is to say no. What you could do is stick to your original assessment and offer to work on the website. You could even take a finders fee or commission if you can find them a decent team to work on their in-house system.

I don't want this to demotivate you or anything. It's just a warning from my own experience of doing this very thing 3-4 years into my own dev career.

"Building a massive system that would automate their daily operations" means not only you need to build it, but maintain it, and be reponsible what happens in the future, as the system be critical for this company.

Without experience, eg. how to construct the agreement you may stuck with it with no proper income, and no time for other projects.

My advice is not to do it :)

UPDATE:
Just had a meeting with the company. I explained to them that it is best if they recruit a full time software engineering team and let them build it and maintain it. I also recommended that they utilize some mature saas products and integrate it with their erp.
I will be focusing on building saas products from now on.

Be careful. Consider Outsource the project. Use a project management tool and start working on your team. You can find a lot of good freelancers out there. So focus an project management and split tasks to your team.