Well, $50 for the modchip installed isn't a bad deal. Even a solderless one can be a little tricky if you don't know what you're doing.
As for the HD? That's quite simple to install. If you have ever put a HD in a PC, you can put one in the XBOX. So, you can find a good deal on a HD on here, you could save some money. Of course, you'd have to reload all of the XBOX stuff onto the new HD, so having him do it would save you that effort.
(and so you know, replacing the HD will result in you getting banned from Xbox Live)