not sure if you're on PC or console, but it's quite easy to get plats by trading with other players. check out warframe.market, there are some stuff quite easy to farm, but mostly prime parts.
Syl
joined 2 years ago
the base mods are usually serration (rank 6 is good enough when you start), multishot and ele damage. viral (tox + cold) or corrosive (tox + elec)
attack speed and crit damage is usually the way to go with melee. But like @Retrikaethan, heavy efficiency can be quite good as well. I would go with attack speed, crit damage and ele or heavy efficiency.
My man Frost!! love him with Thermal Sunder and Archon Vitality.