House Manderly (the North) image - Seven Kingdoms: Total War mod for Total War: Attila

House Manderly is the richest house in the north after the Starks because they controle the largest harbour in the north so it's absolutely a house that afford to armor their soldiers properly.

All Houses have full suit plate for the knights, remember that the show is not a good reference on how these houses really do look when GRRM pictured them as he was writing asoiaf. Every house in the north that has Cavalry wears plate armour, the foot soldiers all have mismatched armour no matter if they're a stark or a manderly or an Umber, in Lore terms it really is down to the personal wealth of the individual if they have good armour or not - and no they're not uniformed... Lore terms. But you are right that Manderly is the second richest, arguably the first.

What I'm getting at is, the above Manderly screenshot really suits the Manderlys for the show. You can't say that Manderlys look like marines although I can see why people would think that with their symbol and fleet, but the actual marines look like marines not its foot soldiers and cavalry - if you were to ask GRRM what the Manderlys really look like he wouldn't have a straight answer for that question, they would just look northern which is very bland armour, but due to Manderly history with them coming from the Reach his men may dress a little more colourful, but other than that - in very basic lore terms most northern Houses simply look the same, the only way you could possibly really tell them apart is from maybe cloth colours or sigils, but even that's unreliable because not all men necissarily wear their houses colours.