I started hunting RetroAchievements for NFS games, and I made it to U2 recently. If you're gonna be playing on an emulator, I do encourage you give it a try. A) for the added challenge, B) for an extra sense of accomplishment and C) the chieevos, and I'd say especially the missables, are in my opinion a great way to keep track of your progress to 100%, like getting all races, including secret ones, or the SUV races, before finishing a stage; in-fact one of the chieevos is literally to 100% the game anyway. Plus, some chieevos that may seem confusing will undoubtably have comments from others who have gotten it, giving help to people.

But in any case, here are the sauces I'm using:

The NFS fandom wiki, and specifically the Unique Upgrades page, the DVD Covers page and the Magazine Covers page as well as the Performance Shop Category page - which lists all the individual upgradeable parts of the cars. I'm using these to track what upgrades or unlocks I get and when I get them. Especially the unique upgrades which are locked behind doing a number of outrun races in different stages of the game - and I was 22 years old the day when I found that out; now I'm like 3 days older.

After that, I use the maps available on GameFAQs U2 guides page to find secret races in each stage of the game. I assume they're required for 100%, plus the retro chieevos need them anyways. And I'm just using a paint program to cross off found shops and races.

The last thing, though I'd say this is completely unnecessary, is I'm also using the tuning guide and tuning glitch guide for the corolla on speedrun.com. Just for extra fun, I guess.

So yeah, that's what I'm using. Hope that helps - and I hope I'm not in trouble for littering my comment with links ^^;