Honestly, Don't Change Much
Warforged are widely considered the second or first strongest LA 0 race in 3.5, fighting with Humans (valiantly) for the top spot. If Humans didn't get much of a boost, 'Forged don't need to either. At most, I'd say to alter them for a net 0 if and only if humans got an ability score bonus. However, you may need to alter the text on the Living Construct subtype to make it compliant with any alterations in terminology that Pathfinder made - changes that are subtle enough to be difficult to reference off of the top of my head.
The only other thing to keep in mind is the idea of Favored Class bonuses, but I think that might be a little outside the scope of this question.
Composite Plating and Heavy Plating are intended to count as armor.
Keith Baker, original creator of Eberron and lead designer for the Wayfinder's Guide to Eberron, addresses this question (and many others) in the WGtE FAQ on his website:
Does the Warforged Heavy Plating Integrated Protection work with the Fighter’s Defense Fighting Style? Unclear on if (armor) means you’re “wearing armor” or not.
The intent is while you are using darkwood core you are not considered to be wearing armor for purposes of game effects such as Unarmored Defense, while you are considered to be wearing armor when you’re using one of the plating modes. However, as written Integrated Protection doesn’t allow you to benefit from the Defense Fighting Style. This won’t be changed until people have a chance to provide feedback on the race mechanics as they stand.
Baker points out that the feature doesn't specifically clarify that Composite Plating or Heavy Plating count as armor - but the parenthetical after each one reflects the intent, which is that Darkwood Core counts as being unarmored and that the other two options count as being armored. This is relevant for features like the Defense fighting style (which requires you to be armored), or the monk's Unarmored Movement feature (which, as the name implies, requires that you be unarmored).
In another response, he also says:
When a warforged is using the darkwood core (unarmored) mode and have light armor proficiency, can they choose whether they are considered to be armored or unarmored?
No. You are always considered to be unarmored when using darkwood core, and always considered to be armored when using plating.
So you can choose which of the three modes to use (depending on what armor you're proficient in), but you can't choose whether that mode counts as being armored.
...but as currently written, they don't count specifically as medium or heavy armor.
Baker also answers a related question about what kind of armor the Integrated Protection feature counts as:
Composite Plating and Heavy Plating both say “Armored” but do they count as medium and heavy armor respectively? Specifically does a warforged Barbarian in Heavy Plating mode lack access to the benefits of rage?
As written, the current version simply specifies that plating counts as armor, not that it counts specifically as heavy or medium armor. So in this version, a warforged barbarian can rage while in the heavy plating mode… though they will have to acquire the heavy armor proficiency before they can use heavy plating.
This may change in the future, but as of right now, a warforged barbarian can gain the benefits of Rage regardless of what defensive mode they're in. However, they can't benefit from features that forbid any armor at all while using Composite Plating or Heavy Plating.
Best Answer
It's hard to say
In the Wayfinder's Guide to Eberron, the specific subraces have guidance on build and weight difference. That being said, only the Juggernaut subrace lists a weight ("up to 450 pounds") out of the three subraces provided in the book. The Skirmisher mentions that they are "lean and designed for speed."
The only other weight references I could find came from previous editions in the form of the 4e Eberron Player's Guide and the random height/weight tables from 3.5e's Eberron Campaign Setting. The 4e sources mentions an average weight of 270-300 lbs. and the tables from 3.5e can generate a weight between 272 lbs. and 318 lbs.
Update:
With the release of Eberron: Rising From the Last War, we have an official weight/height generation for 5e Warforged that is the same as the 3.5e one. That being said, they also removed the subraces for Warforged in this publication so there sadly isn't more clarification on this.