The limit will be 20GB, or whatever the storage allocation is for the plan you choose. There is a ~7MB filesize limit if you use the DreamHost WebFTP to upload, but this limit is caused by the default DreamHost PHP installation. The limit does not exist if you use a 'real' FTP client.
None that I am aware of.
This will depend on the particular server you end up on. My server ('Bixel') has been very good, I have only noticed a few (very) short periods of downtime in the last few months. However, some of the other DreamHost servers do not seem to be as reliable. To be fair, DreamHost seems to be very good at eliminating causes of instability as they become aware of them.
There was a (very) recent thread about this, see the following link...
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