If i use my main character in this module and use the stuff found in the tomb, is it considered cheating? I mean my level 5 fighter now has a +3 full plate breastplate and +2 Tower shield? It says on the description that it's meant for levels 5-10, Does this seem strange?

Equipping your character with custom items isn't cheating as long as you keep within the level restrictions.
It's not the puritan's way of playing perhaps, but unless you're giving your character a level 61 sword to wear or use console commands to boost the strength to 45, it's all legal.

That said, although it doesn't apply to your items, it's not necessarily proper. It's very well possible to make items with low level requirements which are nonetheless very powerful. Or you could be adding OnHitCastSpell properties (walmost always 'wrong' if still legal), or carrying a lot of spellcasting items around so you'll basically have infinite uses/day, or...

There's nothing wrong with what are basically quite modest items like yours though.

The level restrictions on items is determined by their gold cost; the level range for modules is what is advised based on the enemies you'll face. Usually the two are more or less balanced, but it needs not be the case.