There are two parts to this, but most likely, no I won't buy the stock. If you told me I had to buy one stock, it might be Rivian, but I think that stock picking is for me, a fool's errand. If Rivian IPOs, I will indirectly own it through index investing, described below.
It is not because I don't believe in Rivian. It is because I don't believe in my ability to pick stock/issuer winners from losers. In fact, the data suggests professional stock pickers cannot do that over time. All of my investing is for retirement, with a smaller amount for kids' educations. I also like to keep my life simple where I can, and passive index investing does that. I basically get market return year in and out, minus about 0.05 - 0.10 % (I haven't done the math because fees are a bit different for each index fund). Google Bogleheads when you have down time on a rainy day and nothing else to do.
1). I have a so-called Boglehead investing philosophy. I have decided how much I want to try to invest each year, how much should go to stock mutual funds, and how much to bond mutual funds. This means I don't end up buying any stock, just mutual funds that take my money and buy stocks (you could do ETFs) that replicate the entire market.
As to stocks, I made two decisions: one all U.S. index fund, and one all-world (ex US) index fund. The funds try to replicate, more or less, total equity growth of public companies. It gets a little more complicated because I have been in the working world for almost 20 years, and issues like IRAs, 401ks, alphabet soup bla bla come into play, but it is fairly simple. I just make a few choices in life, work, save when I can, and invest to my targets if I can.
Even with an asset allocation, I spend about 30 minutes to two hours per a year thinking about investments. Most of that is gathering accounts and balances and seeing how far the percentages (domestic vs international vs bonds) are off my target. Then I set my investing in the future to help offset that, or makes sells and buys in accounts that do not have tax consequences (e.g. selling stock in a 401k and then buying a bond in the 401k if my bonds are too low vs the others). You can find neat rebalancing sheets on line, including I think at the bogleheads wiki page.
2) Even if I wanted to, I actually have some restrictions on trading for work that would exclude Rivian. I started working at a great place a few years ago with this restriction, but it does not impact my investments because they are all mutual funds. I do have ability to request an exception, but it doesn't really jive with my philosphy.
I admit my admiration for Rivian could lead me to seek an exception for some small amount that I would personally view as a 'fun' pick, but it really goes against my personal investment philosophy.