To add another thoughts on Leman Russ loadouts.
First, the standard battle cannon is quite good. Its targets of choice span from primaris marines, death guard marines, any bikes, up to medium tanks, monsters, and is useful even against knights.
It is not a specialized variant, but at least it is fairly good against a lot of targets.
Vanquisher has been badly nerfed in 8th edition. In previous editions, its gun had a mean strength of 11.5 , which made it quite OK in its niche anti-tank role.
Now the battle cannon is better on every aspect. Shame !
Same remark applies to multi-melta sponsons, with an aggravating factor being their very limited useful half range of 12". Don't bother.
Exterminator auto-cannons are another cool old standard which translated very badly to 8th edition rules. Too expensive, and low
AP, to be worth it.
Just take a pair of Hydras instead, for a little more points than an exterminator, at least you can inflict some damage to numerous nasty flying units.
Eradicator nova cannon ? Maybe. If you are short on points, they can provide some anti-heavy infantry firepower that negates cover bonuses.
Its best target are Eldar rangers,
SM scouts,
MEQ in cover. Not great, but at least can achieve something.
That said, the best Leman Russ kit to choose is the demolisher one, not the standard kit. This is the kit I would recommand for beginners on a budget.
You have access to Punisher, Executioner, Demolisher turrets, that are all quite specialized, but present good efficiency in their respective specialities.
The 3 weapons can be magnetized easily, which is nice.
The weapon mount does not share its shape with the battle cannon kit, which is unfortunate. Fitting a battle cannon on it will require some serious conversion.
The demolisher is comparable with the battle cannon in anti-tank firepower. But has better efficiency against tough targets like knights, plageburst clawler...
The punisher is insanely useful against hordes (tyranid critters, orks, poxwalkers, plaguebearers, cultists etc...) .
Too short ranged for your taste ? --> Consider the Vostroyan doctrine.
Not a huge drawback if you have boards correctly loaded with
LOS blocking terrain anyway.
The Executioner is quite good too, and this
AP-3 !! It is quite cheap for what he can do.
Combo with tank commander order "gunners kill on sight".
Add plasma sponsons and go wreck some hard targets !
As for sponsons, apart from multi-meltas which I discussed earlier, there are two main options :
If you take a tank commander, you should opt for plasma cannons. With rerolls of

they are almost always worth the points.
Note that overcharging is not safe against -1 to hit (or worse) units, even with rerolls.
Otherwise, on a heavy support
LRBT, triple heavy bolters are nice, dice rolling is fast to resolve.
Special mention to secondary weapons configuration with triple flamethrower (aka fireball Leman Russ).
Now that heavy flamer is cheaper, it has something to offer.
On a short ranged tank, that has to come close to the enemy, it can be a deterrent to charges, or give the tank a standing chance against assaulters.
The tank will be less likely to get charged by a random unit taking an opportunity to tie it in combat.
Still a bit pricey for what it offers though. I think this heavy weapon needs to be at the same price as in 6-
th 7-
th editions (10 points).