I am looking for some advice and guidance in fixing up my old mountain bike. It's a Diamondback Apex from the early 90's and it is completely original. I have not ridden it in probably 5 years. It has been in the garage the whole time and only looks dusty no rust anywhere. I aired up the tires and went for a spin and it seemed fine just not as comfortable as I remember. I plan on taking it to a lbs for a complete tuneup but then I wanted to add some upgrades to make it more comfortable. I plan on using it about 80% on paved streets and 20% on hard packed trails. At this point I am not doing anything seriously offroad until I am in better shape. I would like recommendations of brands to look into since I don't know a damn thing about any bike items. Also I would like to know if I am throwing good money after bad in fixing this one up or should I just bite the bullet and replace it.

Here is alist of things I would like to replace:

Tires (They are the original SMOKE tires, I would like something that is smoother on paved streets but could go offroad a little)

Handlebars (I think I want riser bars for comfort)

Grips (Old ones are dry and ripping)

Seat (I wouldn't mind one that is a little more comfortable with out looking like a beach cruiser seat)

I would consider changing the stem so the handlebars are higher but the brake wire connects to the stem and all of the stems I have looked at recently wouldn't work. Would I have to replace my brake?

I know this is alot to ask but any and all help is appreciated.