What are you looking to get out of this ship? Even a "free" ship can become very expensive when you factor in the time it takes to grind all the marks, EC and dil you need.
I like polaron but you need consoles that boost flow to get the most out of it and a plasmonic leech really should be a part of almost every DPS build. Cannons are good but I prefer beams because of FAW 3 being a lt commander ability. For a polaron DPS build I'd go with either the full Iconian set for the damage and defense boosts or the Quantum Phase set with the Krenim core for the flow and EPS boosts.
I am looking for advice, to maximize my performance, either DPS and survivability.
I prefer cannons. I'd like to know which space set is the best for this ship with polaron DHCs and why. As you said, you recommend full Iconian set, but why? Is full Ico more effective than 3pc Ico and Romulan advanced prototype engine that offers attack patterns buff?
Plasmonic leech is needed, but I don't have enough EC for it. I'm looking for cheaper variants... I have just 10mil. EC but 500k dil (partially refined) and 3000 marks in each rep in average. Except plasmonic leech, are there any other important consoles that I can afford?
Start grinding fleet credits, the most important consoles for a build like this are the Spire and Embassy fleet console. You'll want Vulnerability Locators with +pol to boost crth and polaron damage and Plasma-Generating Weapon Signature Nullifiers with +flow for the plasma damage, lower threat gen and to boost polaron's drain. For now use the best console you can find from drops or cheap on the exchange and work towards using all fleet console for tac and science and a Conductive RCS console with EPS, the Assimilated Module and Plasmonic Leech for engineering. There are ways to get around using the Leech but then you need traits the come from zen and lobi ships.
The only real boost the Romulan engine give you is the attack pattern boost but the Iconian engines give you defense, energy damage resistance while at full impulse and the 4 piece set gives you an ability that boosts the whole teams damage, hull and flight speed.
Unfortunately there's nothing cheap if you want to get the most out of a ship, if you just want enough to get by in advanced STFs I can recommend a cheap build but high DPS is expensive.
If you were using all energy weapons I'd say get rid of the Chronometric Capacitor but with it boosting polaron and torp damage and the build you're using I'd keep it. Fleet weapons are good, advanced fleet crtd/dmgx3 are one of the best weapons in the game right now, maybe the best if you don't want to invest the time and dil it takes to get a crafted dmgx3/pen set. As for polaron types, the only ones I'd match are the DHC and turret from the set you're using and use different polaron types for the rest. They will all be boosted by your +pol tac consoles and you will gain the benefits of the other procs. If you want to keep it relatively cheap I'd use the set weapons with advanced fleet for the rest. I don't know a lot about torps but on a cannon build you'll be forward facing so I would only use one torp, I'd stick with the Krenim for the set, and put a second turret on the back.