Cardboard box grow

I wouldn't use card board box. 600 W LED does get hot. The bottom of the box will eventually get wet. This is an accident waiting to happen. Best to start with a small grow tent. And you will need air circulation otherwise plant can't get enough carbondioxide. Too much moisture will leads to mold. And you should start with low smelling strains. And these are usually easier to grow. I recommend any strain with Northern Light or Jack Herrer in it. Don't go for award winning strain just yet.

You wont need a filter, unless your trying to keep it stealth. It only stops smells from going out if its carbon based and small derbies like bugs and dust from getting in if your referring to a intake filter on the air exchange. I grow in a make shift homemade tent out of Styrofoam, I don't use filters and come harvest and the weeks its in heavy flower my place smells like fresh pot, I like it, not to strong. Air circulation if key though, place a fan or two in there. Humidity will probability be your biggest problem but the mini fans have saved me pushing new air in and moist air out. helps prevent mildew and moulds. Also if the case is cardboard, think about putting Mylar "safety blankets are one cheep option" on its walls or painting them matte white. Helps light bounce around. I use the white paint option in mine, not as good as the Mylar but the blankets are cheep and rip/dint/wrinkle to easy then become less effective, Professional diamond style Mylar is way too expensive for a homemade budget tent.

air circulation should be there,it is more important than smell.if you are concerned about fitting a filter in there, there are some products like ONA Gel which can suppress the smell up to a point.1 plant wouldn't create much problem. or have an incense in the other corner of the room or somewhere else,this can make a better job than ona gel.

Is it purely for smell purposes then to have a filter running or for air circulation primarily too? I was thinking of doing a weaker smelling strain and having only 1 or 2 plants wouldn't of thought smell would be too bad as neighbours expect it from me smoking all the time aha. Would appreciate any other advice too!

Depends, can you grow legally? If yes, then it is not necessary; if not, then it is a must hace.

It will become necessary in flower yeah, possibly even before if you have a particularly strong smelling strain

I have also got mylar reflective sheeting to line the inner box