Your question is very widely scoped, making it difficult to reply to, but I can provide my thoughts on at least the following part of the question:
I have a 401k plan with T. Rowe Price, should I use them for other investments too?
Using your employer's decision, on which 401k provider they've chosen, as a basis for making your own decision on a broker for investing $100k when you don't even know what kind of investments you want seems relatively unwise to me, even if one of your focuses is simplicity.
That is, unless your $100k is tax-advantaged (e.g. an IRA or other 401k) and your drive for simplicity means you'd be happy to add $100k to any of your existing 401k investments. In which case you should look into whether you can roll the $100k over into your employer's 401k program.
For the rest of my answer, I'll assume the $100k is NOT tax-advantaged.
I assume you're suggesting this idea because of some perceived bundling of the relationship and ease of dealing with one company & website? Yes, they may be able to combine both accounts into a single login, and you may be able to interact with both accounts with the same basic interface, but that's about where the sharing will end. And even those benefits aren't guaranteed. For example, I still have a separate site to manage my money in my employer's 401k @ Fidelity than I do for my brokerage/banking accounts @ Fidelity. The investment options aren't the same for the two types of accounts, so the interface for making and monitoring investments isn't either.
And you won't be able to co-mingle funds between the 401k and non-tax-advantaged money anyway, so you'll have two different accounts to deal with even if you have a single provider. Given that you'll have two different accounts, you might as well pick a broker/provider for the $100k that gives you the best investment options, lowest fees, and best UI experience for your chosen type/goal of investments.
I would strongly recommend figuring out how you want to invest the $100k before trying to figure out which provider to use as a broker for doing the investment.