Regarding roller rockers...
Much of the friction in the valve train is at the rockers. The stamped rocker rotates against a "half moon" metal retainer. This is a little simplistic but... There is probably .5 sq inch of friction surface per rocker * 16 = 8 sq inches of metal to metal friction surface. Take two pieces of steel 2.83 inches square (8 sq inches), put a few drops of oil between them, apply about 200 PSI and rub. This is where all the friction occurs.
Roller rockers remove this friction.
Comp Cams, I think, does offer a "poor mans" version of roller rockers. It is a roller bearing retainer that fits the stock stamped rocker arm. Not as good as full roller rockers, but probably an improvement. Last I saw they were about $125.
Good roller rockers should be available starting at the $200 range from Jegs or Summit (Crane Cams). Also consider used. The roller rocker does not take a lot of abuse when it comes down to it, they are re-usable. You might find a good set of used rockers at
www.coral.net
Many HP ratings on cams also include roller rockers. Check the fine print.
You can get away with stock valvetrain components, but you will be defating some of your work. I think roller rockers and springs are worth the cost. Check with the cam manufacturer, they will list a recomended rocker and spring setup,also if new srings are required (I think new springs might be required for the E-303 cam).
Unless your putting in Nitros or blower the 19# injectors are OK. 24# would be an upgrade that would also require a new fuel pump and fuel regulator.
I have not checked this out yet, but roller rockers may also require new - taller - valve covers. You might be able to stack gaskets to get the clearance. If you go to 1.7 ratio rockers (not recomended) you WILL need taller valve covers. The stock Explorer valve covers are the lowest ones I have seen. Then if the taller valve covers bump into the intake manifold you could add a phoenelic spacer to lift the manifold and keep the intake air cooler.
To install youself you might have most the tools you need in your tool box right now. HOWEVER, you MUST use a Cam Degree tool to set the cam timing properly. You might be able to beg,borrow or rent one.
Erson is now owned by MrGasket. They make a good product. Make sure it is a roller cam setup.
You can get by with the stock fuel pressure regulator. An adjustable one would be a nice upgrade.
In the future you will also want to consider a larger throttle body. Yes the explorer TB was from the GT or cobra mustang at 65mm. From experience I will say that larger is better at the top end ( I wish my 5.0 with ported heads had a larger TB) Remember you are making that engine breath better. Breathing through a straw won't help.
I don't mean to increase the costs beyond reason, but there is usually more involved than just swapping in a new cam with all other components stock. Will it work, probably. But you may not get the performance you expect. If you are trying to control cost, and who isn't, You can phase items in. I would suggest you do the entire valve train now and get the other goodies like throttle body, MAF, and chip programming as add-ons when you can afford them.
P.S. Just one more item... You will need a new set of manifold, head bolts and gaskets, no way around it. MrGasket, or Felpro gaskets and ARP bolts make good kits. Expect to pay another $150 - $200 for these replacement parts.
Seriously, I can't wait to tear into my motor again. Plus, I need to take my radiator out anyways so I can get my fan shroud deal back in. 
i will probably have the cam up for sale soon, as my engine plans have changed. i just want something mild/streetable now, and my cam is too big (plus its boost/nitrous specific).
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