“Armour protects the wearer by increasing their global hits if it covers the majority of the torso and at least one other location”
So the absolute minimum from my reading of those rules would be a breastplate that covers just over 50% of your torso. Which they tend to do due to wrapping around the side.
In terms of getting the measurements right, if you don’t have a gambeson, try measuring yourself in the thickest coat you have to give yourself an estimate.
Breastplate is often less comfortable without a backplate ive found, not sure if it balances out somehow or ive just been unlucky with breastplates 
Most of my advice already covered though!
In terms of the armour on that site, I can only judge from the images. The quality of the design is very poor from what I can see, with a number of red flags that are strong indicators of problems.
For example, with the cuirass you linked, it is cut too wide in the chest, which will likely bruise your arms when you fight, especially if you plan to use a sword. I am also really dubious about how they have fitted those tassets, because they have have fitted a version of what look like 17th Century cavalry tassets to a 15th century (ish) breastplate, without including the linking plates that would normally be there to make it sit properly and not bruise/cut your thighs.
The arm defences look to be made in a way that has the upper arm sitting entirely wrong, so if you had the elbow and lower arm sitting properly the upper won’t be defending you - instead it would clash with the cuirass, limiting movement.
If you do want to go for budget armour like this, you can sometimes make it work if you apply a bit of cunning. For larp, you can get the image of the knight in shining armour you are looking for if you use chainmail for the body, covered by a tabard in your colours. Then have some nice shoulder/arm/leg armour in plate which will give you the look you want. Body armour is absolutely the hardest to make work, especially if it is badly designed, so you sidestep it by hiding the one place where you have armoured in chain rather than plate.
My advice if you want to spend some money and get the hit points but also look good is, and what i wish id done
Buy chain. Steel not ali. Rivited under the arms
Epic armouries do some for 80 quid.
Buy polyurethane gorget and spaulders. Light weight but look like steel and you can get some nice looking ones
Wyrmwick do a nice set for around 140.
Or take a chance.
Safe bet is buy chain. Add to it later.