Can I use a hardtail for MTB trail riding?
First and foremost... Yes, you can definitely ride trails on a hardtail and it will help you get lots of technical skills that a full suspension bike wouldn't give you, unless you jumped class (enduro) on a trail bike. Plus, it will be way more comfortable riding in the city. Secondly, given your budget, i would suggest you didn't make a custom one. Buy a new one. There are lots of good ones on that budget.