I know most of this is situational, but should I generally buy more than one MG? I usually buy one when the game first starts so I can get early point advantage, but after that I try and focus on more mobile units like Panzergrenadiers. Would double/triple MG be more helpful for Penal/AT Rifle rush?
The Mg is osts strongest unit. It is fairly vital component of combined arms. I does depend on game mode but I would say you are going to end up with 2 at least - one in your main battle line and one covering a key flank/resource. Overinvesting can backfire as flamecars drive past them or US mortar smokes them or mortars chew them up or light armour slowly kills them whilst you lack grens with faust to destroy/delay them.
Panzergrenadiers are very good if you get the positioning in your favour but very expensive - glass cannons. Generally speaking in infantry vs infantry the allies are going to come out on top in the end unless you have suppression from mg's to help you. For example when the brits get their 5 man squads and double vickers with a bit of vet your infantry will just melt (as will mgs that are not in green cover!)
In small games you might balance quite carefully when your 2nd/3rd mg comes ie. get one then invest in grens before getting one more before you tech to next tier. I might build another one later in things are going well to hold ground I have taken or create defense in depth, i.e. an mg behind my 'main' battleline mg. This prevents flanks on it AND allows you to retreat mgs from the field for healing/reinforcement without leaving an open door for the enemy to walk in and harvest your paks and mortars.