Menem would have still lost, although depending on how much rejection Lopez Murphy had (the guy was known for trying to cut university funding and failing at that, and also for being part of the UCR, which had failed miserably in handling the 1998-2002 crisis when in power). As to the consequences...
I think this kind of WI relies on the "Great Man" vs the "Historical trends" visions of history. So, Lopez Murphy is a right wing economist more aligned to the so-called "pro-business" side. But he'd be facing the same challenges Nestor Kirchner had:
The debt default: we can all believe Lopez Murphy would be willing to pay more to the creditors. But he'd still have to negotiate, because paying all the accumulated interest plus the ongoing debt services wasn't possible. The variety of the debt would still make it a gargantuan bureaucratic effort. There would still be holdouts, and probably a Rights Upon Future Offers clause in whatever exchange bonds his team negotiates, which means Argentina remains in default for a long time (ten years after the first deal remains a good possibility)
He might have favored FTAA, but he would still had have to contend with the local economic sectors opposed to it as well as the Brazilian opposition. As long as Brazil opposed it, FTAA wasn't going to happen. And if it comes to it, who would any Argentine president in 2005 favor? A USA disinterested in the region, or a growing Brazil that's growing like mad, increasing trade with Argentina and looking at leading the region?
He can be called an austerity hawk, but then again, so was Nestor Kirchner, who ran surpluses even with the economy growing at very high rates
He would still have to deal with European countries (mostly Spain) seeking to arrest the Argentine torturers from the last dictatorship and face the choice of refusing to extradite them (or keep them from being put on trial abroad) on sovereignty grounds, or deal with the issue domestically and repeal the 1980s amnesty laws
He would have had to deal with the vast amounts of poverty the crisis had left in its wake, because even if the economy was growing a lot, there is no way it was going to grow fast enough to solve that in less than a decade.
And specially if Menem resigns from the race, he'd still need to contend with the lack of political legitimacy "winning" an election with less than 24% of the vote and coming out second carry. He will need something to sustain his moral authority.
Finally, if he reelects in 2007, he would have had to deal with the fallout of the 2008 crisis without access to the financial debt markets, which probably means he'd have to retort to a combination of monetary emission while keeping the value of the US dollar relatively low in order to contain inflation, just like Cristina Kirchner had, or turn to the IMF with the negative political and economical outcomes attached to it.
So in short, not too different really. Maybe he'd actually be less fiscally conservative in order to invest more in infrastructure and power generation. He wouldn't take an antimilitaristic instance, so we should see slightly better equipped Armed Forces, but with less emphasis on local production of military gear. He'd have a better relationship with the USA by virtue of not receiving Fidel Castro at his inaguration, not humiliating Bush when rejecting the FTAA and not pointlessly antagonizing them. But that won't really make a difference during his first term. Frankly, it wouldn't make much of a difference until Obama's second term anyway and by that time Lopez Murphy wouldn't be president even if he reelects.
Now, if Menem managed to win the run off by virtue of being less impopular than Lopez Murphy, that would be fun! Not necessarily good, but fun. He'd be under the same constraints and would try to deliver the image of stability and modernization. If he can deliver in that context, that's another matter.
