Best of luck to your friend! Looks like there are some fantastic ideas and amazing conversions out there already to inspire them. Ultimately this is going to be a matter of the proportions your friend feels comfortable with. A German 88mm cannon is going to look pretty slender compared to a Vanquisher, and might be more suitable as a counts-as lascannon. Generally speaking it looks like the 1:35 scale models can work, but you'll need to patch in
GW weapons (or convincing substitutes) to pull it off. This is reflected in some of the better conversions above, particularly the KV-2 / Demolisher and T-34 / Hellhound above...
Having said that, I was too stingy to buy the vehicle upgrade kits for my Leman Russes, and substituted with a 1/35th Scale Academy US Machine Gun set. The Ma Deuce (M2) at 1/35th looks pretty slender compared to the
GW heavy stubber but it's hard to doubt what it's doing there and what it represents.