Yes - any gigabit switch will be okay. If funds permit might I suggest you consider an 8 port switch - you will be surprised as to how soon you can fill them up.
As to your ATA query, it depends on whether you wish to continue using your existing telephone setup and just add the ATA device to bridge it to your LAN or whether you are considering a wholly new DECT phone system.
As to the former, I use a Grandstream HT802 ATA
which supports 2 VOIP lines, is about £40 and is very capable, but frankly for the novice it has so many configuration options it might be somewhat overwhelming.
If you do decide to purchase one, I found this page
helpful in configuring it.
If updating the whole system, the already suggested Gigaset N300A IP has the capability of connecting up to 6 individual VOIP lines as well as hosting an old fashioned PTSN line. The actual device is about £65 plus handsets on top. I use a mixture of Gigaset handsets with mine ranging from circa £30 to £70 each. This IMHO is much easier to configure.
I'm sure there will be others more knowledgeable than, around shortly with alternative suggestions - especially on the ATA front.