I cannot tell you whether or not that particular computer is a good deal - don't know enough about all the systems like that to compare. I hope others here can advise, or maybe even start a separate thread in the appropriate forum for advice on choosing a machine. I tried to find out whether the HP unit has an IDE port on its Foxconn mobo - the HP site even tells you the mobo model number - but I cannot find that info on either HP's or Foxconn's website. So I can't advise whether this machine has the IDE port on the mobo that you would require in order to connect your old IDE drive.
It's not very complex, actually. And since your plan is to buy a new computer fully assembled and functioning and then add your old drive inside it, much of my previous post won't matter to you at all. The last paragraph is nearly everything you need to know. About the only thing to add is IF your new computer has its optical drive connected to an IDE port, there are a few details to check. To tell: the mutli-wire ribbon cable for data from the mobo to the drive is about 2" wide for an IDE device, with a connector on the end that has 40 holes - 2 rows of 20. On the other hand, if it is a SATA unit, the ribbon cable is only 7 wires less than 1" wide, and in that case you can ignore the optical drive and concentrate solely on connecting your IDE older unit.
So, the simple case: your optical drive is SATA and there is no IDE device installed, although there is an unused IDE port on the mobo. You will need your old drive and 4 screws to mount it inside the case, an empty drive bay for this, an 80-conductor IDE cable, and one 4-pin Molex power connector (coming out of the PSU) free to use. The 80-conductor IDE cable is a very common one, but there are two variations. Most have three connectors on them: a blue one on one end that plugs into the mobo port, a black one on the other end that plugs into the Master device, and a grey one in the middle that plugs into the Slave device. They come in various lengths, so choose one long enough to reach without strain, but not too long. Each connector has little bulges on one edge and at least one hole blocked off, you can only plug them in one way. Some cables omit the middle connector to save money, and that's OK if you know you'll never need the second device, but I always get the one with all 3 connectors just to preserve options. If you don't have a Slave unit, you just don't connect to the middle grey connector.
With only one IDE device to attach, you use the little diagrams on the drive to set its jumpers so it is the Master, and I bet your old drive already is set this way. After mechnaical mounting, you plug in its power connector and the IDE ribbon cable black end connector.
IF you have a computer that already is using an IDE optical drive, you will need to make a couple small adjustments. In that situation, the optical drive will have its jumpers set to be a Master or to the "CS" position (for "Cable Select"). When you add your old HDD it should take over as Master, so the optical drive's jumpers will need to be changed to the Slave position. Then you'll arrange the ribbon cable so the black end connector goes to the HDD, and the grey middle connector goes to the optical unit. Not complicated at all!
When you add your old HDD you should check that the machine does not try to use it for booting. I fully expect this will all be OK, but just check to be sure. To do this you need to enter the BIOS Setup screens. With most machines that is done by holding down the "Del" key as you turn it on. Then you wait while the first few lines of stuff show up on screen, and it will pop up the opening menu of this Setup system. There will be prompts at the bottom and on the right for how to move around with cursor keys and how to change stuff if you need to. It is all menu screens you work with. Move to the area where the Boot Priority Sequence is specified. It should be set to use the optical drive as the first boot device, the go to the SATA hard drive as the second one, and NO other units tried after that. If you have to change something to get this, use the correct keys to Save and Exit. If no changes, just plain Exit Without Saving will do nicely. The machine should reboot and detect both your old and new drives, and Windows should show you the old drive and all its files in My Computer.