Are condensing units still unreliable?

No.

How do purchase costs compare between the a combi and a condenser

You may be getting confused. The "combiness" of a boiler has no relationship
to the "condensingness" of the boiler.

A condensing boiler is more efficient than a non-condensing type, but
produces a certain amount of steam plume, particularly when the flue is
short. With a loft mounting, this plume is likely to be much less of a
nuisance. The emissions from the boiler are actually much cleaner than a
traditional boiler. It is just that the temperature is much lower, so it is
more visible that people get annoyed. Non-condensing boilers will shortly be
banned.

A combi boiler replaces your hot water cylinder with instantaneously heated
water at mains pressure. This gives excellent shower performance, recovers
space that would be taken by a cylinder, is more energy efficient and
cheaper to install.

However, there are major disadvantages too. It takes longer to get hot water
at the tap (some boilers have systems to remove this disadvantage at the
expense of some energy efficiency). It produces much lower potential flow
rates, leading to slow bath filling. It will only work if there is good
pressure and flow rate available from your mains water supply.

If you decide that the slow bath filling is an issue, then you should
install a storage system. When designing a system from scratch, you should
really install a mains pressure system (provided that the mains is up to
it). You have a choice of installing a mains unvented cylinder, or a heat
bank. Both will provide oodles of hot water at mains pressure, with many
times the flow rate of a combi.

An unvented cylinder is likely to be easier to get hold of. A larger
proportion of installers will have heard of them and know how to install
them. They provide the highest flow rate.

A heat bank has the advantage of inherent safety and doesn't need the
elaborate precautions and regular servicing that an unvented cylinder needs.
This is because it doesn't store the hot water at mains pressure, but
instantaneously heats the incoming water from the stored zero pressure
water. This makes an explosion much less likely, so there is no need to use
an approved installer, or have regular servicing for the safety valves and
vessels.

Some boiler manufacturers produce boilers with integral unvented cylinders
or heat banks. If you see a "combi" quoted with more than 40kW, there is a
good chance that it is not actually a combi at all, but a preplumbed
combined heat bank and boiler system relabelled at a combi for marketing
reasons.

Any/all advice most welcomed!

So, you need to answer the following questions:

1. How many baths and showers do you have?
2. How many people live in the house?
3. Does anyone prefer baths to showers?
4. What is the maximum flow rate at your kitchen sink? (use a bucket and
stopwatch to measure).
5. How happy were you with your old system when it worked? (I presume it was
probably a gravity hot water cylinder fed by gravity circulated water from
the back boiler).

Christian.