Effectively you want a Holy Avenger with the Sun power? Since you said you want a Longsword then the only powers of the sun sword that are useful is the Sun Power. Not sure what it would cost but it should still be below any +10 weapon.
Now if you added in the +4 against evil and the acting as short sword but still damaging like bastard sword that would be too much I think.
Such a custom item probably wouldn't scale well using the written custom item rules.
It would almost certainly be Epic. (At least by the 3.5 definition, PF has no epic itemms)
It also depends on which of the enhancements stack. The most reasonable would be a +5 Evil Bane (i.e. +2 enhancement +2d6 vs. evil) Also, how it would function in the hands of a non-Paladin. Which is kind of moot because I assume this is for a Paladin. The other abilities would work as advertised.
I wouldn't even try to price it, if you want to add it to your campaign, make it some ancient thing that can't be made anymore.
I would place such a sword between a +10 and +12 equivalent.
I was kind of curious at this as what the title says.
Is it possible to combine the holy avenger with sunbladeand how effective would it be as a longsword.
As with all custom magic items, the only real answer to that is: "Anything is possible if your referee allows it." That, in the end, is your largest hurdle.
If they say yes they will then simply ask them how much one would cost, or if it would need to be researched as a special magic item.
If your asking for a more detailed breakdown of cost and effect then the other posts in this thread are a good referance point. The two weapons if combined into one would be extremley potent, possibly bordering or exceeding balanced in all but the most high powered games.
As has been stated, given what the sunblade power does, it is irrelevant if it looks like a longsword or not. It is wielded like a short sword (which anyone who can actually use any sword can use) and hits like a bastardsword.
The sun blade is priced at 50,335 gp. We can determine through reverse engineering that the base weapon is a +2 bastard sword valued at 8,335 gp. Thus the sunblade's special abilities are valued at 42,000 gp.
The holy avenger is similar. Through simple reverse engineering, we can determine that the holy avenger is a +2 cold iron longsword, valued at 10,315 gp, while the holy avenger itself costs a whopping 120,630 gp; and thus are worth 110,315 gp.
Thus a sunblade holy avenger would likely be priced at 152,315 gp plus the cost of the magic weapon. So a +5 cold iron bastard sword that is also a holy avenger sunblade, would cost a whopping 204,650 gp.
Now, the holy avenger was probably priced with a 30% discount (due to class/alignment restriction) on it's increased powers, so if you want the enhancement bonus to scale (such as in the case of the +5 bastard sword becoming a +8 holy bastard sword) then you would likely want to tally the difference between a +5 weapon and a +10 weapon, and then add the difference with a 30% price reduction to the final tally.
But if you just wanted them "as is", and just added together, 152,315 gp plus the cost of the magic weapon is probably your best bet. You can use these guidelines if you want to apply the effects to some other sort of weapon as well (such as a holy-avenger greatsword or rapier).