The odds of getting whips in RS Gold the 75-125 kill range is possible although not tipacal. possible. There's a better chance of getting one within a range of 1-540 kills. But, you may not be able to get one. Also, if you ask anyone about the reason they train slayers it's almost always to get the biggest drops. For example, whips from abby deamons, sols made from ice strykewyrms etc, for example.
Even if they slay for these drops the majority of their profits won't be derived from them, especially on ice wyrms which have stunning drops but the SOL is a mere extra. The 1/540 is derived from the average dropping (mite be 520 I'm not sure) but it is due to players who report how many kills are required to earn a whip, sure, you can get the range you want, but this isn't the drop rate.
Helm of Neitiznot It's a nice balanced helm. Verac's or Torag's helm for greater defensive stats or a fighter hat to increase offensive stats (about similar stats to the Rune full helm but with bonus points in the Stab, Slash and Crush attack styles)
Guthans Plate - Decent, Fighter torso gives a nice Strength bonus so it's a great option in case you're not able to afford Bandos Also, I'm not completely certain about this but I believe Torag's (and Dharok's) gives slightly better defensive statistics and is around 650k so it's worth it in my opinion.
Your current setup is fine However, I'd suggest something that doesn't degrade. Dragon Platelegs with Fighter Torso, for example. Dragon Platelegs aren't expensive, while that Fighter Torso only costs a couple of hours to Buy OSRS Accounts purchase. Or Bandos armour if you're a very wealthy player.