I have an LG Optimus Slider (Virgin Mobile LGVM701) that is stuck on the LG logo screen. The phone was given to me so I have no idea what happened. However, I would like to try to fix it IF possible.You can install a rom with ADB, however I suspect the phone may be defective given your description.
You can find used OV's for $60 if you look around, new ones on sale for not much more if you can find them.
I almost did the same as you, but make sure you know exactly what you bill will be with T-Mobile. Many of their plans have taxes on top of the amount indicated, which can be another $15 a month.