Having repaired (or tried to repair, in one case) HP and Dell, I would avoid.
Until the last machine, I'd built my own for about 20 years. I couldn't match the price of the one I bought: I checked motherboard spec, graphics card, etc and was and remain very pleased with it.
I specified a 400 watt power supply which I thought would be enough. It was for a year or so! Go for the biggest you can get without paying silly money. Too many available watts aren't wasted. Too few cause all sorts of problems.
The important thing is to decide what you do and don't want it to do. That will decide (for example) whether or not you want multiple hard drives and a RAID arrangement, or a super-expensive water-cooled graphics card, or Blu-ray etc.
I looked at a new machine a month or so back, and for the tower and innards I reckoned on spending £550. I could have paid over £1200 from the same supplier.