You asked about reliability so you seem to care about the potential for data loss. You didn't clearly specify your goals and requirements so it's hard to give you specific advice but with a budget of $400 you could easily buy multiple hard drives for redundancy. You could then for example put multiple hard drives in a so-called RAID configuration, with RAID standing for redundant array of independent disks. There are simple RAID configurations that let you mirror data between two drives. If one of the drives in the RAID were to then suddenly fail you would still have a copy of your data. RAID is not a substitute for backups though! If something happens to the machine that runs the RAID (lightning strike, theft, fire, etc.) then there is a good chance that you will still lose all of your data on that machine.

The standard for professional backups is the 3-2-1 rule: have 3 copies of your data on at least 2 different types of media and have at least 1 copy off-site; for consumers this may be overkill though. Instead you could for example opt to have a RAID in your home that is the primary way for you to store your data and to have one additional HDD at your workplace or at a friend's house as an off-site copy. If you decide to do this, remember to encrypt the HDD that leaves your control to prevent third parties from accessing your data. 

When it comes to the specific machine to put the HDDs in the most obvious choices are either a desktop PC or a so-called NAS (network-attached storage) server which is a dedicated machine serving data to other machines on your network. You can either build a NAS server yourself (which requires some basic system administration skills) or you can use a product like a Synology DiskStation that is targeted more towards non-experts.

Alternatively you could store your data or one of your backups in the cloud where you can reasonably assume that professional backup standards will prevent you from losing your data. You will need to decide for yourself however if you trust the cloud provider with your data. For large data volumes cloud storage can also be a problem in terms of bandwidth.

I hope I was able to help you. In case I misunderstood your intent or if you have any other questions, feel free to let me know!
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