I have thousands of documents which are around 450 MB in size that I’d like to make into one project. Can Scrivener handle this without getting sluggish?
The sum total size of all of your documents within Scrivener doesn’t usually matter (250MB is the sum of all your thousands of files, right? You don’t have thousands of files which are 250MB each…? 
). There are some performance impacts that can be worked around when it comes to project backups and the Scrivenings view of many documents in one editor, but you should be fine if you learn to deal with those two issues.
Are these documents mostly of the kind that are converted for editing by Scrivener upon import (.doc, .rtf, .txt…) or are they pdfs, image files, multimedia, etc…? If they are text to be imported and converted, then you should avoid viewing all 250MB’s worth in one Scrivenings session; that will take quite a while to load into the editor window. Instead, view them in outline or cork board mode. Also, if any of them are of significant length (greater than… I dunno… 50,000 words I guess…) I suggest splitting them up into smaller chunks. You can always view those chunks in a scrivenings view to see the whole thing at once.
The automatic backups are your other concern. Using the .zip compression option may make the process unpleasantly long, and you may not want it to happen when you close the project and want to shut down the computer immediately. Also, if you set your backups (Tools->Options->Backups) to go to a file sync folder like Dropbox or the like, keep in mind that your quota of space may be consumed quickly if you have it set to keep more than a handful of backups per project. Still, I wouldn’t recommend avoiding backups altogether; maybe use the option that makes good use of File->Save (or ctrl-S), so you can decide exactly when to create a new backup.
What Robert said 
There’s also the question of how good a fit you will find Scrivener to be for yourself and your project. It’s not something that one can really predetermine theoretically, hands-off.
I would suggest that you download the free Scrivener trial download. Full features, good for 30 days (not necessarily consecutive) use.
literatureandlatte.com/trial.php
Do the tutorials. Help > Interactive Tutorials. It may ask you a location or name to store a work copy under. The manual is available via Help and also from the site if PDF format.
literatureandlatte.com/supp … #Scrivener
Then load some or all of your project in and work through the major phases you anticipate going through from start to finish, to get hands-on experience and assure you likely will not encounter any major surprises later. Doesn’t have to be your entire project… but enough to represent it realistically and provide solid proof-of-concept.
Don’t hesitate to plunge in. Limit your investment and the scope of the experiments and trial run you undertake and, worst case, the only thing you will be out is some time.
You can do searches on the forum via search field in upper right corner.
There are articles about Scrivener out on the Internet and books/ebooks about Scrivener available on Amazon and elsewhere.
Scrivener is different, otherwise no point in its existence.
It does have a learning curve, as does everything else.
Plunge in.
I like automatic backups, but I hate times I enter a huge project to check on something and then it has to back up when I exit. Sure would be nice if it knew no changes had been made and backup wasn’t necessary.
This is something we’re planning to refine in a future version. It’s a complicated by the different types of files that change within the project just by you opening it and clicking around, even if you don’t make any textual changes to the project. Every time there’s a selection or the editor size changes or you hide the inspector, the project updates with that information. Those changes may not not warrant a backup, but other changes to the same files may.
To avoid the delay with large projects, especially if you’re using them more for reference at this point than regular work, you could choose File > Back Up > Exclude from Automatic Backups and then just be sure to run the backup manually when you are working in it. File > Back Up > Back Up Now, or its main toolbar icon, will run the automatic backup routine even when the project is excluded from the automatically triggered backups, and File > Back Up > Back Up To… lets you select a name and location for the backup, separate from the auto-backups.